GIFT  OF 

AUVHOR 


o4 


°d 


HOW   TO   KNOW   THE   STARRY 
HEAVENS 


^.  . 


FIG.   1.  —  SOLAK  PROMINENCES,  BY   TROUVELOT,    OF  HARVARD 
OBSERVATORY 

The  white  circle  represents  the  size  of  the  Earth  on  the  same  scale. 


HOW   TO   KNOW 


THE 


STARRY   HEAVENS 

AN    INVITATION    TO    THE 

STUDY  OF  SUNS  AND  WORLDS 

BY 

EDWARD    IRVING 


WITH  CHARTS,  COLOURED  PLATES,  DIAGRAMS, 

AND  MANY  ENGRAVINGS  OF 

PHOTOGRAPHS 


NEW   YORK 

FREDERICK   A.  STOKES  COMPANY 
PUBLISHERS 


COPYRIGHT,  1904 
BY  FREDERICK  A.  STOKES  COMPANY 


All  rights  reserved 
Published  in  November,  1904 


THE  UNIVERSITY  PRESS,  CAMBRIDGE,  U.  S.  A. 


"Everybody  should  study  astronomy.  It  is  the  most  delightful 
of  all  the  sciences.  It  is  the  most  inspiring  of  all.  It  lifts  and 
broadens  the  mind.  It  rouses  the  imagination,  and  the  imagination 
is  the  most  God-like  of  human  faculties,  because  it  is  the  most 
creative.  Let  no  one  be  deterred  by  the  superstition  that  it  is 
necessary  to  be  a  mathematician  in  order  to  understand  and  enjoy 
astronomy.  You  can  let  the  mathematics  of  the  subject  severely 
alone  and  yet  Jind  inexhaustible  pleasure  and  advantage  in  astro- 
nomical study.  It  is  because  they  were  compelled  to  begin  at  the 
mathematical  end  of  the  subject  that  hundreds  of  thousands  of 
graduates  from  schools  and  colleges  have  virtually  no  knowledge  of 
astronomy.  Mathematical  gifts  are  rare,  but  they  are  not  essential 
to  the  enjoyment  of  astronomy."  —  GARRETT  P.  SERVISS. 


PREFACE 


THIS  volume  is  not  so  much  a  text-book  on  Astronomy,  as 
an  invitation  to  read  text-books  on  that  subject.  In  other 
words,  it  is  a  careful  selection  of  the  most  typical,  interesting, 
and  instructive  facts  and  theories  concerning  the  Universe 
around  us.  The  author  has  endeavoured  to  describe  and  illus- 
trate these  in  such  a  way  as  to  attract,  interest,  and  inform  the 
general  reader.  But,  though  intended  primarily  for  beginners, 
every  effort  has  been  made  to  avoid  offending  those  who  are 
further  advanced,  by  sensationalism  or  a  want  of  proportion  and 
accuracy.  The  comparisons  and  illustrations  used  are  the  re- 
sult of  many  years'  study,  and  have  been  successfully  used  in 
lectures  and  classes.  They  may  interest  some  who  are  well 
acquainted  with  the  facts  of  astronomy,  but  have  not  looked  at 
them  from  the  same  standpoint. 

Many  interesting  and  important  astronomical  methods,  prin- 
ciples, and  facts  have  been  left  out  of  this  volume,  to  avoid 
overcrowding  and  confusion.  The  main  object  of  the  work  is 
not  so  much  to  describe  individual  worlds,  as  to  enable  the 
reader  to  realise,  as  far  as  possible,  what  the  Universe  itself  is 
like.  In  other  words,  it  is  to  give  a  bird's-eye  view  of  the 
celestial  forest  from  a  general  and  philosophical  standpoint, 
so  that  the  individual  trees  may  be  afterwards  examined  more 
at  leisure.  Until  such  a  bird's-eye  view  has  been  obtained,  the 
learner  is  apt  to  be  confused  by  the  details.  As  the  old  saying 
has  it,  he  "  cannot  see  the  wood  for  the  trees." 

When  such  a  general  view  has  once  been  obtained,  the  details 
no  longer  confuse,  and  text-books  that  were  formerly  thrown 
down  in  disgust  become  luminous  with  the  ever-growing  interest 
that  rightly  belongs  to  the  physical  sciences. 


viii  PREFACE 

The  figures  given  in  this  work  are  mostly  round  numbers. 
They  do  not  claim  absolute  accuracy,  but  at  the  same  time 
every  effort  has  been  made  to  avoid  serious  errors. 

The  distance  of  the  nearest  star  has  been  given  as  about 
9,000  times  as  great  as  that  of  Neptune.  It  is  quite  possible 
that  further  investigations  may  result  in  other  figures  being 
adopted.  But  even  if  it  should  be  changed  to  8,000  or  10,000, 
the  comparisons  used  will  still  serve  to  illustrate  the  relative 
dimensions  of  the  visible  Universe. 

The  author  gratefully  acknowledges  the  kindness  of  Professor 
K.  G.  Aitken  and  other  members  of  the  staff  at  the  Lick  Ob- 
servatory, in  reading  the  manuscript  and  making  suggestions 
which  have  materially  helped  to  perfect  the  work. 

Thanks  are  due  to  many  who  have  given  advice,  suggestions, 
corrections,  and  information;  also  to  those  who  have  granted 
the  reproduction  of  valuable  photographs  and  drawings.  Among 
these  are : 

The  late  Dr.  E.  Keeler,  Director  of  Lick  Observatory, 
California. 

Dr.  W.  W.  Campbell,  Director  of  Lick  Observatory,  California. 

Dr.  A.  0.  Leuschner,  Director  of  Students'  Observatory, 
Berkeley,  Cal. 

E.  L.  Larkin,  Director  of  Lowe  Observatory,  California. 

C.  Burckhalter,  Director  of  Chabot  Observatory,  Oakland,  Cal. 

G.  E.  Hale,  Director  of  Yerkes  Observatory,  Wisconsin. 

E.  C.  Pickering,  Director  of  Harvard  Observatory,  Massa- 
chusetts. 

W.  H.  M.  Christie,  M.A.,  Astronomer  Eoyal,  Greenwich, 
England. 

E.  Walter  Maunder,  F.K.A.S.,  Greenwich,  England. 

Besides  giving  an  account  of  well-known  and  indisputable 
astronomical  facts,  the  author  has  touched  upon  certain  specu- 
lative theories  which  cannot  yet  be  proved  by  either  experiment 
or  observation.  The  most  that  can  be  said  for  them  is  that 
they  give  a  reasonable  explanation  of  a  large  number  of  ob- 
served phenomena,  and  must  therefore  contain  a  certain  amount 


PREFACE  ix 

of  truth.  They  also  help  to  give  us  a  better  idea  of  the  Infinite 
and  Eternal  Drama  in  which  our  little  Earth  is  playing  its 
obscure  and  ephemeral  part.  The  reader  is  not  asked  to  accept 
these  theories  if  he  can  explain  the  observed  phenomena  by 
more  probable  speculations  of  his  own.  But  he  must  beware 
of  adopting  theories  which  conflict  largely  with  ascertained 
facts. 

It  only  remains  to  be  said  that  these  speculations  have 
everywhere  been  carefully  distinguished  from  those  facts  which 
are  so  well  proved  as  to  be  practically  indisputable. 

This  volume  is  intended  to  be  the  first  of  a  series,  by  the 
same  writer,  dealing  with  the  sciences  of  astronomy,  geology, 
biology,  and  sociology.  These  four  were  grouped  together  by 
the  late  Herbert  Spencer  under  the  name  of  the  Concrete 
Sciences.  Though  the  vast  importance  of  these  subjects  is 
now  generally  recognised,  many  otherwise  educated  people  are 
lamentably  deficient  in  them.  This  is  very  unfortunate  for  the 
individuals  concerned,  for,  however  learned  a  man  may  be  in 
all  other  subjects,  it  is  impossible  for  him  to  be  truly  broad- 
minded,  philosophical,  and  cosmopolitan,  without  some  knowl- 
edge of  these  Concrete  Sciences.1 

A  general  lack  of  scientific  knowledge  injures,  not  only  the 
individuals  themselves,  but  also  society  at  large.  In  spite  of 
the  great  advances  made  in  all  directions  during  the  last  century, 
there  are  still  many  imperfections  remaining  in  our  systems  of 
government,  of  administrative  justice,  of  national  education,  and 
in  our  entire  social  and  moral  organisations.  These  imperfec- 
tions are  largely  due  to  the  fact  that  many  of  our  statesmen, 
lawyers,  teachers,  doctors,  and  preachers  are  deficient  in  the 
above-mentioned  sciences.  Let  us  hope  that  during  the  present 

1  As  the  philosophical  A.  Zazel  truly  says : 

"  Astronomy,  geology,  biology,  and  sociology  together  form  an  impregnable 
bulwark  against  the  inroads  of  superstition.  And  where  the  seeds  of  that  deadly 
mental  disease  have  been  already  sown,  these  sciences  form  an  infallible  antidote 
and  cure." 


x  PREFACE 

century  this  ignorance  may  be  removed,  so  that  our  upward 
progress  may  no  longer  be  impeded  by  the  erroneous  ideas  that 
have  been  dragged  up  with  us  from  the  flat  world  in  which  our 
ancestors  imagined  themselves  to  be  living. 

The  second  volume  will  deal  with  the  history  of  the  Third 
Planet  in  our  System,  from  its  nebulous  birth  to  the  advent  of 
Man.  Its  title  will  probably  be  How  to  Know  the  Earth's 

History. 

EDWARD  IRVING. 

BERKELEY,  CAL.  (U.  S.), 
October,  1904. 


CONTENTS 

CHAPTER  PAGK 
I.    APPARENT  MOTIONS   OF   THE    HEAVENLY    BODIES   AS 

SHOWN  BY  OBSERVATION 1 

II.    RIVAL  THEORIES  TO  EXPLAIN  THE  APPARENT  MOTIONS 

OF  THE  HEAVENLY  BODIES 15 

III.  PRINCIPLES  UTILISED  FOR  MEASURING  THE  UNIVERSE  26 

IV.  SOME   PROBLEMS  USED  IN  CELESTIAL  MEASUREMENTS  39 
V.    THE  CHARIOT  OF  IMAGINATION 53 

VI.    DIMENSIONS  OF  THE  UNIVERSE  .........  68 

VII.    SOME  MORE  DIMENSIONS 77 

VIII.    THE  PRINCIPLES  AND  APPLICATIONS  OF  THE  SPECTRO- 
SCOPE       87 

IX.    A  STAR-SPANGLED  BANNER 102 

X.    CONSTRUCTION  OF  THE  UNIVERSE 114 

XL    SOLAR  ARCHITECTURE 125 

XII.    A  REELING  WORLD 136 

XIII.  KEPLER'S  THREE  LAWS 155 

XIV.  GALILEO'S  LAWS  OF  MOTION 163 

XV.    NEWTON'S  LAW  OF  GRAVITATION 167 

XVI.    ANCIENT  COSMOGONIES,  AND  THE  NEBULAR  HYPOTHESIS  178 
XVII.    THEORIES  AND  DISCOVERIES  MODIFYING  THE  NEBULAR 

HYPOTHESIS 190 

XVIII.    MODIFICATIONS  OF  THE  NEBULAR  THEORY     ....  205 

XIX.    THE  MESSENGERS  OF  HEAVEN 219 

XX.    LARGE  AND  SMALL  WORLDS 233 


x»  COiNTENTS 

CHAPTER  PAGE 

XXI.  IGNEOUS  FORCES  ON  THE  MOON  AND  ELSEWHERE    .     .  243 

XXII.  LUNAR  GEOLOGY  AND  GEOGRAPHY 257 

XXIII.  INHABITED  WORLDS 265 

XXIV.  SIZE,  IMPORTANCE,  SPEED,  AND  DURATION      ....  283 
XXV.  CONCLUSION  290 


APPENDIX  A.  —  FACTS  AND  FANCIES  CONCERNING  MATTER  .     .  295 

APPENDIX  B.  —  THE  GREEK  ALPHABET 301 

APPENDIX  C.  —  THE  LUNAR  CRATERS 302 

INDEX  .    ,  309 


ILLUSTRATIONS 

FULL-PAGE  ILLUSTRATIONS 

FIGURE 

1.    Solar  Prominences,  by  Trouvelot,  of  Harvard  Observatory 

frontispiece  in  colours 
FACING  PAGK 

7.  Northern  Star-Trails 14 

8.  The  Dipper,  or  Great  Bear,  at  Intervals  of  Six  Hours     ...  14 

24.  Sun,  Showing  Spots  and  Faculse 52 

25.  Group  of  Sunspots 58 

26.  Solar  Flames  and  Corona,  as  Seen  During  Eclipse  of  May  28, 

1900.     (In  colours) 56 

27.  Eruptive  Prominences 58 

28.  Solar  Corona.     Eclipse  of  May  28,  1900 60 

29.  North  Polar  Streamers  of  the  Corona.     May  28,  1900      ...  60 

30.  Mercury,  the  First  Planet 62 

31.  Venus,  the  Second  Planet 62 

33.    Mars,  the  Fourth  Planet 62 

35.  The  Zone  of  Asteroids  Between  Mars  and  Jupiter       ....  64 

36.  Jupiter,  the  Largest  Planet 64 

38.    Saturn,  the  Ringed  Planet 66 

41.  Lick  Observatory  on  Mount  Hamilton,  California 70 

42.  Main  Entrance  and  Great  Dome,  Lick  Observatory     ....  70 

43.  The  Thirty-Six-Inch  Refractor  at  Lick  Observatory    .     .          .  72 

44.  Eye-Piece  of  the  Great  Lick  Telescope 74 

45.  Yerkes  Observatory,  Williams  Bay,  Wisconsin 80 

46.  The   Forty-Inch   Refractor   of  the   Yerkes  Observatory:   tin- 

Largest  in  the  World 80 

47.  Milky  Way  Surrounding  Messier  II 

48.  The  Star-Cluster  Messier  II 84 

53.  Laboratory  and  Celestial  Spectra.     (In  colours) 94 

54.  Tele-Spectroscope 

55.  The  Mills  Spectrograph  at  Lick  Observatory 96 

56.  Chief  Lines  in  the  Solar  Spectrum  (Herschel)  . 

57.  Part  of  the  Spectra  of  Four  Red  Stars  (Hale  and  Ellerinan)      .  100 

58.  Star   Spectra    Showing   Displacement  of  Lines  Due  to  Star's 

Motion  in  Line  of  Sight 


xiv  ILLUSTRATIONS 

FIOUBE                                                                                                                           FACING  PAGE 

59.  Coloured  Double  Stars.     (In  colours) 108 

60.  Star-Cluster  in  Hercules 112 

61.  Part  of  the  Milky  Way  in  Sagittarius 114 

62.  A  Rope-like  Nebula  in  Cygnus 114 

63.  Spiral  Nebula  in  Triangulum 116 

64.  The  Great  Nebula  in  Andromeda 118 

65.  A  Spiral  Nebula  Seen  Edgeways 120 

66.  The  Ring  Nebula  in  Lyra 120 

67.  The  Trifid  Nebula  in  Sagittarius -122 

68.  Great  Nebula  in  Orion 124 

69.  A  Typical  Sunspot 128 

70.  Solar  "  Flames  "  or  Prominences 132 

75.  The  Solar  Corona  During  the  Eclipse  of  July  29,  1878     ...  134 

76.  Theoretical  Section  of  Solar  Photosphere 132 

79.  Equatorial  Mounting  of  the  Crossley  Reflector,  Lick  Observatory  150 

80.  Meridian  Circle 150 

82.  Drawing  an  Ellipse 158 

84.  Mount  Lowe  Observatory,  in  Southern  California 168 

85.  Spiral  Nebula  in  Ursa  Major  (M  81) 172 

91.  Dumb-Bell  Nebula 192 

92.  Nova  Persei,  1901.     Showing  Movement  of  Surrounding  Nebu- 

losity.    Lick  Photographs " 196 

93.  Spectra  of  Nova  Persei,  Showing  Changes 200 

94.  The  Star- Cluster  Omega  Centauri 204 

100.  A  Celestial  Messenger  Approaching  a  Star 222 

101.  Brook's  Comet,  1893 222 

102.  Comet  1903  C 222 

106.  Donati's  Comet,  1858 228 

107.  Comet  Rordame,  1893 228 

109.  The  California  Meteor  of  July  27,  1894 232 

113.  Full  Moon,  Showing  Radiating  Streaks 244 

114.  The  Moon,  at  First  and  Last  Quarter 248 

117.  Clavius  and  Tycho 252 

118.  Theophilus,  a  Lunar  Crater-With-Cone 254 

122.  Mare  Crisium,  a  Lunar  Plain 258 

123.  Lunar  Apennines  and  Alps 258 

124.  Copernicus 260 

125.  Schickard  and  Wargentin 262 

126.  Ptolemy,  Alphons,  and  Arzachel 264 

127.  Twelve  Views  of  Mars  . 268 

128.  Disc  of  the  Sun,  August  12,  1903 274 


ILLUSTRATIONS  xv 

CHARTS 

Chart  A.    The  Northern  Heavens    |                                               PACING  PAGE 

Key  to  Chart  A.     (In  colours)        J 282 

Chart  B.    The  Equatorial  Constellations.    For  Spring  Evenings  ] 

Key  to  Chart  B.     (In  colours) .  j      -290 

Chart  C.    The  Equatorial  Constellations.     For  Summer  Evenings  ) 

Key  to  Chart  C.     (In  colours) °.  }  294 

Chart  D.     The  Equatorial  Constellations.     For  Winter  Evenings  ) 

Key  to  Chart  D.     (In  colours) }  30° 

Chart  E.     Eastern  Half  of  Moon.     (In  colours)  ) 

Chart  F.     Western  Half  of  Moon.     (In  colours)  \ 804 

Chart  G.     The  Constellation  Figures 308 

ILLUSTRATIONS  IN  THE  TEXT 

FIGURE  pAGs 

2.  Umbrella-Apparatus   for  Illustrating   (Apparent)   Star  Move- 

ments        5 

3.  The  Earth,  Showing  Relative  Positions  of  Apparatus  when  Used 

at  Equator,  Poles,  etc 7 

4.  Umbrella- Apparatus  Modified  for  Illustrating  Apparent  Move- 

ments of  Sun  and  Planets 8 

5.  Circles  of  the  Celestial  Sphere  with  World  in  the  Centre      .     .  9 

6.  An  Adjustable  Equatorial,  Suitable  for  any  Part  of  the  World  13 
9.    The  Ptolemaic  System 19 

10.  The  Tychonic  System 20 

11.  Copernican  System 23 

12.  Relative  Positions  of  Earth  and  Sun  at  the  four  Seasons  ...  24 

13.  Orbits  of  Mercury,  Venus,  and  Earth 28 

14.  Estimating  Distances  with  the  Eyes 31 

15.  Surveying  from  a  Base-line 32 

16.  Daily  Positions  of  Earth  and  Moon       ....*....  37 

1 7.  Arc  of  Circle 40 

18.  Chord  of  Arc  . 41 

19.  Sine  of  Angle 42 

20.  Measuring  Width  of  River 43 

21.  Measuring  Distance  of  Moon 46 

22.  Arc  of  Circle 47 

23.  Sine  of  Angle 50 

32.    Terra,  the  Third  Planet,  and  Its  Satellite  or  Moon     ....  63 

34.    Relative  Sizes  of  Earth  and  Mars 64 

37.    Relative  Sizes  of  Jupiter  and  Earth 65 


xvi  ILLUSTRATIONS 

FIGURE  PAGE 

39.  Relative  Sizes  of  Saturn  and  Earth 66 

40.  Relative  Sizes  of  Neptune  and  Earth 67 

49.  A  Prism  and  its  Spectrum 90 

50.  A  One-prism  Spectroscope 91 

51.  Section  of  a  One-prism  Spectroscope 92 

52.  A  Compound  Spectroscope 93 

71.  A  Solar  "  Cloud  "  of  Glowing  Hydrogen  (Professor  Young)  .     .  131 

72.  The  Same  Region  35  Minutes  Later  (Young) 132 

73.  The  Same  Region  35  Minutes  Later  (Young) 133 

74.  The  Same,  15  Minutes  Later  (Young) 133 

77.  Diagram  Illustrating  Zodiac 137 

78.  Diagram  Illustrating  Precession  of  Equinoxes  and  Advance  of 

Perihelion 143 

81.    Alt- Azimuth  Mounting  for  Small  Telescope 153 

83.    An  Elliptical  Orbit,  Divided  into  Twelve  Monthly  Parts  .     .     .  159 

86.  Original   Nebula,  after   its  Rotation  has  Produced  a  Disc-like 

Form 184 

87.  Nebula  with  Outer  Ring,  left  behind  by  Contraction  and  Conse- 

quent Quickening  of  Rotation 185 

88.  Central  Condensation  Surrounded  by  Rings 186 

89.  Rings  Collapsing  into  Planets,  and  Central  Condensation  Turn- 

ing to  a  Luminous  Sun 187 

90.  Solar  System  as  it  is  now 188 

95.  Earth-tides,  if  the  Day  and  Month  Were  Equal 213 

96.  Acceleration  of  Moon  by  Forward  Pull  of  Earth-tide  .     .     .     .  214 

97.  Loop  in  Apparent  Path  of  Mars 215 

98.  Diagram  Showing  Cause  of  Loop  in  Apparent  Path  of  Mars      .  216 

99.  A  Celestial  Messenger  on  a  Journey 221 

103.  Parabolic  Orbit  of  a  Free  Comet 227 

104.  Elliptical  Orbits  of  Captive  Comets 228 

105.  Tail  of  a  Comet  near  Perihelion 229 

108.   A  Meteor  Bursting  in  the  Atmosphere 230 

110.  Relative  Sizes  of  Planets 234 

111.  Relative  Sizes  of  Sun,  Jupiter,  and  Earth 235 

112.  Relative  Sizes  of  the  First  Four  Asteroids  and  the  Earth's  Satel- 

lite   236 

115.  Section  of  Earthly  Volcanoes 251 

116.  Section  of  Lunar  Volcano  in  full  Activity 252 

119.  Section  of  Mountain  of  Exudation 254 

1 20.  Section  of  Mountain  of  Elevation 254 

121.  Section  of  Lunar  Crater  with  Cone                       255 


HOW   TO   KNOW  THE 
STARRY  HEAVENS 


CHAPTER- 


APPARENT   MOTIONS  OF  THE  HEAVENLY  BODIES  AS 
SHOWN  BY  OBSERVATION 

"  Appearances  are  deceptive."  —  Old  Saying. 

"Ne  jugez  pas  selon  1'apparence,  raais  jugez  selon  la  justice." 

—  Fourth  Gospel,  vii,  24  (Segond). 
"  Things  are  not  what  they  seem."  —  Longfellow. 

SUPERFICIAL    APPEARANCES 

BEFORE  describing  the  Universe  as  it  is,  I  wish  to  say  a 
few  words  about  the  Universe  as  it  seems.  We  shall  then 
be  better  able  to  judge  as  to  the  reasonableness,  or  otherwise, 
of  the  various  theories  which  have  from  time  to  time  been 
brought  forward  to  explain  the  celestial  phenomena  which  are 
going  on  around  us.  It  may  be  well  also,  before  dealing  with 
the  dimensions  of  the  Universe,  to  give  a  very  brief  account 
of  the  methods  used  by  astronomers  to  enable  them  to  ascer- 
tain the  distances  and  dimensions  of  those  celestial  bodies 
which  are  within  a  measurable  distance  of  our  World. 

The  conclusions  at  which  modern  astronomy  has  arrived  are 
not  those  which  would  naturally  occur  to  the  first  observers  of 
the  heavenly  bodies.  The  conditions,  indeed,  are  such  that 
superficial  observations  always  lead  to  wrong  conclusions.  To- 
day, in  most  of  our  so-called  civilised  countries,  the  people  in 
general  take  it  for  granted  that  the  Earth  is  a  planet  going  around 
the  Sun.  Many  of  them  have  also  heard  that  the  stars  are  far-off 
suns,  floating  in  practically  empty  space.  Yet  not  one  person 

l 


2        HOW   TO    KNOW   THE   STARRY   HEAVENS 

in  a  thousand  truly  realises  what  these  statements  mean.  They 
are  merely  hearsay,  accepted  in  childlike  faith,  as  some  of  the 
ancients  accepted  the  statement  that  the  Earth  is  supported  by 
a  number  of  elephants  standing  on  the  back  of  a  big  turtle 
whose  legs  reach  all  the  way  down  ! 

Bu.t  in  those  Countries  where  the  secular  schools  have  not 
familiarised  the  people' with  the  accepted  teachings  of  modern 
astra'nc!ray,.a  man  who  asserts  to-day  that  the  Earth  goes  around 
the  Sun  is  regarded  as  either  a  wag  or  a  lunatic.  If  people 
condescend  to  argue  the  point  with  him,  they  can  overwhelm 
him  with  apparently  good  reasons  for  their  incredulity.  They 
can  not  only  give  plausible  arguments  from  their  own  surround- 
ings and  experiences,  but  can  also  prove  their  case  by  wholesale 
quotations  from  the  writings  of  the  "  inspired "  priests  and 
prophets  of  former  times.  If  he  suggests  that  their  surround- 
ings and  experiences  are  wrongly  interpreted,  they  laugh  him 
to  scorn.  If  he  insists  that  the  ancient  writers  were  ignorant 
and  mistaken,  they  abuse  him  as  an  infidel.  If,  to  avoid  their 
resentment,  he  tells  them  that  the  writers  of  their  sacred  books 
did  not  intend  that  their  statements  should  be  understood 
literally,  they  truly  and  philosophically  reply  that  he  is  wrest- 
ing the  Scriptures  to  his  own  destruction. 

ONLY    FACTS    WANTED 

Seekers  after  truth  should  not  be  satisfied  with  mere  hear- 
say. Those  who  expect  to  get  facts  by  faith  alone  generally 
accumulate  fables  instead  of  facts.  Where  faith  is  relied  on,  it 
is  a  mere  matter  of  where  we  are  born  as  to  what  we  believe. 
Faith  may  possibly  do  no  harm  as  regards  immaterial  or  child- 
ish beliefs,  but  it  is  very  hurtful  when  used  for  material  or 
important  matters,  which  require  intelligent  scepticism  to  enable 
us  to  sort  out  the  true  from  the  false. 

Even  if  by  accident  we  should  get  the  Truth  by  faith  alone, 
it  would  do  us  no  good.  One  of  the  founders  of  Christianity 
told  his  followers  to  "prove  all  things"  and  "  hold  fast  that 
which  is  good  "  (I  Thess.  v,  21).  A  better  precept  was  never 


APPARENT   MOTIONS   OF   HEAVENLY   BODIES       3 

given,  though  many  who  profess  to  walk  in  his  footsteps  do  not 
seem  very  enthusiastic  about  following  his  counsel. 

Those  who  are  looking  for  actual  facts  concerning  the  Uni- 
verse should  therefore  leave  faith  to  those  who  are  satisfied 
with  pleasant  fables  and  flattering  delusions.  They  should 
endeavour,  by  all  the  means  at  their  command,  to  ascertain  for 
themselves  whether  these  things  are  truly  as  represented,  and 
they  should  also  try  to  realise  what  the  facts  of  the  case  really 
involve. 

THE    MUSIC    OF    THE    SPHERES 

As  regards  the  shape  of  our  Earth,  it  is  not  now  necessary  to 
prove  that  it  is  a  sphere.  Many  of  us  have  travelled  enough 
to  satisfy  ourselves  by  actual  experience  as  to  its  general  size 
and  shape.  Even  those  who  have  lived  all  their  lives  in  one 
locality  have  now  plenty  of  positive  evidence  that  the  old 
theory  of  its  being  flat  is  untenable.  As  regards  the  rest  of  the 
Universe,  however,  we  still  have  to  rely  on  observation  and 
abstract  reasoning. 

In  order  to  ascertain  whether  the  sky  is  a  hollow  rotating 
sphere  surrounding  the  Earth,  or  whether  it  is,  as  now  claimed, 
a  boundless  ocean  swarming  with  suns  and  worlds,  let  us 
examine  it  and  the  various  objects  which  appear  to  be  "  fixed  " 
to  it,  or  to  be  wandering  around  on  it. 

The  most  noticeable  of  the  permanent  objects  in  the  sky  are 
known  as  the  Sun  and  Moon. 

The  most  numerous  and  steadfast  are  called  the  "fixed  "  stars. 
They  were  so  named  because  they  do  not  appear  to  change 
places  relatively  to  one  another. 

A  few  objects  which  very  much  resemble  the  stars  in  appear- 
ance are  distinguishable  from  them  by  several  peculiarities. 
For  example,  they  do  not  twinkle  like  the  stars,  but  shine  with 
a  steady  unflinching  light.  At  some  periods  they  shine  very 
much  more  brilliantly  than  at  other  times.  And  they  slowly 
change  their  places  among  the  "fixed"  stars.  For  this  latter 
reason  they  are  known  as  planets,  or  "  wanderers."  The  best 


4       HOW  TO   KNOW  THE   STARRY   HEAVENS 

known  of  them  go  by  the  names  of  Latin  deities  who  were 
formerly  identified  with  them.  They  are  called  Saturn,  Jupiter, 
Mars,  Venus,  and  Mercury. 

Oft-repeated  observations  of  the  heavenly  bodies,  from  differ- 
ent parts  of  our  globe,  long  since  proved  that  they  all  appear  to 
have  certain  definite  and  well-defined  motions  which  have  been 
repeated  over  and  over  again  for  hundreds  and  thousands  of 
years.  There  are,  to  be  sure,  certain  irregularities  in  some  of 
these  motions,  but  close  and  long-continued  observations  show 
that  even  these  irregularities  are  themselves  regular  and  cyclic 
in  their  action. 

STELLAR   MUSIC 

The  most  obvious  of  these  motions  may  be  imitated  by  tak- 
ing two  twelve-ribbed  umbrellas  (real  or  imaginary),  opening 
them  both,  and  tying  their  handles  together,  so  that  the  arrange- 
ment forms  a  kind  of  globe  (see  Figure  2). 

On  the  Equator.  —  If  the  observer  lives  on  the  Equator,  in 
that  hot  circle  of  the  Earth  which  lies  between  the  Tropics,  he 
can  represent  the  apparent  motion  of  the  star-strewn  "  sphere  " 
by  keeping  the  handles  of  his  umbrellas  horizontal  in  a  north- 
and-south  direction,  and  slowly  spinning  the  whole  thing 
around  on  its  handles,  so  that  the  rims  of  the  umbrellas  rise 
in  the  east  and  descend  in  the  west. 

The  names  of  the  various  groups  of  stars  can  be  chalked  on 
the  inside  of  the  umbrellas,  and  the  observer  must  imagine 
himself  standing  (in  the  centre  of  the  apparatus)  on  a  flat 
table  which  prevents  him  from  seeing  anything  below  his  own 
level.  The  chalk-marks  which  are  near  the  rims  of  the  um- 
brellas will  then  seem  to  rise  in  the  east,  pass  overhead,  and 
sink  in  the  west.  Those  farther  north  and  south  will  pass 
more  slowly  over  the  handles  or  "  poles  "  of  the  apparatus, 
which  lie  flat  on  the  central  table  and  do  not  change  their  posi- 
tion at  all. 

So  long  as  the  observer  stays  on  the  Equator  there  will  be 
no  change  in  the  position  of  the  starry  sphere,  which  appears 


APPARENT   MOTIONS   OF   HEAVENLY   BODIES       5 

to  turn  completely  over  in  about  four  minutes  less  than  twenty- 
four  hours.1 

It  is  obvious  that,  if  we  turn  our  apparatus  so  as  to  keep  up 
with  the  stars,  a  fresh  rib  will  pass  the  Zenith,  or  point  over- 
head, every  two  hours  (nearly),  and  that  at  the  same  instant 


AS    USED     ON 
THE     EQUATOR . 


*F 


FIG.  2.— UMBRELLA-APPARATUS  FOR  ILLUSTRATING  (APPARENT)  STAR 
MOVEMENTS 

Face  the  west  when  using  this  diagram.    By  reversing  the  points  of  the  compass  as  here 
given,  and  facing  the  east,  it  will  represent  the  Earth's  real  motion. 

the  opposite  rib  will  pass  the  Nadir,  or  point  below.  Also  that 
one  rib  will  rise  above  the  eastern  horizon  at  the  same  time, 
while  another  will  descend  below  the  western  horizon. 

1  If  it  were  not  for  these  four  minutes'  difference  we  should  see  the  same  stars, 
in  the  same  part  of  the  sky,  at  the  same  time  of  the  night,  the  whole  year 
through, 


6       HOW  TO   KNOW  THE  STARRY   HEAVENS 

At  the  North  Pole.  —  But  if  the  observer  travels  to  the  north, 
the  apparatus  will  not  follow  the  motions  of  the  stars  unless 
he  tips  it  up  by  raising  the  northern  umbrella.  By  the  time  he 
reaches  the  frozen  regions  near  the  North  Pole,  he  will  have 
to  tip  up  the  apparatus  so  much  that  the  handles  will  be  per- 
pendicular. The  southern  umbrella  will  then  be  below,  out  of 
sight,  and  the  chalk -marks  on  the  northern  umbrella  will  turn 
around  the  point  overhead.  If  the  observer  now  holds  his 
watch  overhead,  with  the  face  down,  he  will  find  that  the 
chalk-marks  are  going  the  opposite  way  to  the  hands  of  the 
watch. 

At  the  South  Pole.  —  If  the  observer  returns  to  the  Equator, 
he  will  have  to  turn  the  northern  umbrella  down  again,  and 
when  he  sails  into  the  southern  seas  the  southern  umbrella  will 
have  to  be  tipped  up,  to  represent  the  motions  of  the  stars.  By 
the  time  he  reaches  the  frozen  regions  around  the  South  Pole, 
the  southern  umbrella  will  be  uppermost.  A  fresh  set  of  chalk- 
marks  will  then  turn  around  the  point  over  his  head,  and  they 
will  be  found  to  turn  the  same  way  that  the  hands  of  the  watch 
revolve  when  looked  at  from  below. 

During  these  supposed  journeys  from  the  Equator  to  the  Poles, 
the  axis  of  the  apparatus  will  not  really  le  typed  up  either  way, 
for  the  northern  stick  will  point  to  the  North  Pole-Star  all  the 
time,  and  the  southern  stick  will  be  directed  toward  the  same 
part  of  the  southern  skies  all  the  time.  The  apparent  tipping 
up  and  down  is  due  to  the  fact  that  the  surface  of  the  Earth 
is  not  flat,  but  round,  and  therefore  dips  toward  the  Poles. 
The  annexed  diagram  will  show  this  clearly,  the  large  circle 
representing  the  Earth,  and  the  five  small  objects  representing 
our  umbrellas  in  different  parts  of  the  world  (see  Figure  3). 

SOLAR   MUSIC 

On  the  Equator.  —  Let  us  suppose  that  the  observer  is  again 
on  the  Equator  with  his  apparatus,  and  that  he  wishes  to  follow 
the  motions  of  the  Sun.  It  will  be  necessary  to  put  a  hoop 
over  the  umbrellas  where  the  twelve  pairs  of  ribs  come  together. 


APPARENT   MOTIONS   OF   HEAVENLY   BODIES       7 

This  hoop  will  represent  the  Celestial  Equator.     The  ribs  of  the 
umbrellas  should  be  numbered  from  1  to  12. 

Spring  "  Passover."  —  If  it  is  about  the  20th  of  March,  the 
Sun's  position  can  be  represented  by  hanging  a  small  electric 
light  where  the  equatorial  hoop  crosses  the  first  pair  of  ribs. 
On  turning  the  apparatus  as  before,  the  electric  light  will  rise 
in  the  east,  pass  overhead,  and  set  in  the  west.  While  it  is 


FIG.  3.  — THE  EARTH,  SHOWING  RELATIVE  POSITIONS  OF  APPARATUS  WHEM 
USED  AT  EQUATOR,  POLES,  ETC. 

above  the  level  of  the  imaginary  observer  in  the  centre  of  the 
apparatus,  the  chalk-marks  representing  the  stars  must  be 
supposed  to  be  out  of  sight,  on  account  of  the  greater  brilliancy 
of  the  electric  light.  When  it  sets  in  the  west,  the  chalk- 
marks  above  the  horizon  must  be  supposed  to  come  into  view 
again. 

Autumnal   Equinox.  —  Six   months  later  —  about   Septem- 
ber 22  —  the  arrangement  will  be  the  same,  except  that  the 


8        HOW   TO   KNOW   THE   STARRY   HEAVENS 

light  will  have  to  be  shifted  to  where  the  equatorial  hoop 
crosses  the  opposite  or  seventh  pair  of  ribs.  That  is  to  say,  if 
the  light  was  where  the  first  pair  of  ribs  come  together  in 
March,  it  will  be  where  the  opposite  or  seventh  pair  of  ribs 
come  together  in  September. 


FIG.  4.  —  UMBRELLA-APPARATUS  MODIFIED  FOR  ILLUSTRATING  APPARENT 
MOVEMENTS  OF  SUN  AND  PLANETS 

Face  the  west  when  using  this  diagram.  If  used  north  of  the  Equator,  raise  (N)  till  it 
points  to  the  North  Pole,  and  vice  versa.  The  feathered  arrows  indicate  the  diurnal  mo- 
tion; the  plain  arrows  indicate  the  annual  motion. 

Midsummer  Solstice.  —  About  June  21  the  light  will  be  on 
the  fourth  rib,  but  will  be  some  distance  north  of  the  equatorial 
belt. 

Yuletide  Solstice.  —  About  December  21  it  will  be  on  the 
tenth  rib,  but  some  distance  south  of  the  equatorial  belt. 


APPARENT  MOTIONS   OF  HEAVENLY   BODIES       9 

If  a  second  hoop  be  passed  over  the  umbrellas,  so  that  it  will 
pass  over  these  four  places,  it  will  represent  the  Ecliptic,  or 
annual  path  of  the  Sun  among  the  stars  (see  Figure  4). 

It  will  be  seen  that  the  light  representing  the  Sun  does  not 
go  its  daily  round  exactly  the  same  as  the  chalk-marks  repre- 
senting the  stars.  It  moves  slowly  backward  on  the  second 


SUN 


JUNE. 


FIG.  5.  —  CIRCLES  OF  THE  CELESTIAL  SPHERE  WITH  WORLD  IN  THE  CENTRE 

Only  the  upper  half  of  the  diagram  is  supposed  to  be  above  the  horizon  of  the  observer. 
Face  the  west  when  using  the  diagram.    Those  living  north  of  the  Equator  should  raise  (* 
until  the  axis  (SN)  points  to  the  North  Pole-Star,  and  vice  versa. 

hoop,  so  that  the  average  interval  between  one  "  mid-day  "  and 
the  next  is  nearly  four  minutes  longer  than  the  "  southing  "  of 
one  of  the  chalk-marks  on  two  successive  "  nights."  The  re- 
sult is  that  in  the  course  of  366J  revolutions  of  the  umbrellas, 
which  represent  the  star-sphere,  there  are  only  365J  revolutions 


10     HOW  TO   KNOW  THE   STARRY   HEAVENS 

of  the  light  which  represents  the  Sun.  In  other  words,  the 
Sun,  whose  motions  we  are  trying  to  represent,  creeps  slowly 
back  along  the  Ecliptic,  so  that  in  exactly  one  year  it  has  lost 
one  revolution,  having  gone  completely  around  the  "  star-sphere  " 
to  the  place  where  it  was  twelve  months  before. 

As  the  Sun's  path  is  not  on  a  line  with  the  Equator,  but 
crosses  it  obliquely,  the  Sun  not  only  loses  one  complete 
revolution  in  a  year,  but  also  drifts  to  the  north  and  south 
of  the  equatorial  belt,  which  it  "  passes  over  "  twice  in  each 
year,  at  the  spring  and  autumn  "  Passover  "  or  Equinox  (see 
Figure  5). 


"THE  BURNING  ROW" 

In  the  apparatus  just  used,  the  hoop  along  which  the  light 
slowly  travels  represents  the  Celestial  Ecliptic,  or  path  of  the 
Sun.  This  hoop  lies  over  twelve  sets  of  chalk-marks  repre- 
senting twelve  different  constellations  of  stars.  Each  set  of 
stars  has  a  name  by  which  it  has  been  known  for  several  thou- 
sands of  years.  The  twelve  form  what  are  collectively  known 
as  the  Signs  of  the  Zodiac.  They  are  also  known  as  the  Twelve 
Houses  (or  Mansions)  of  the  Sun.  The  Book  of  Job  (xxxviii,  32) 
mentions  them  under  the  name  of  the  Mazzaroth. 

It  takes  the  Sun  a  solar  month  (a  little  longer  than  a  lunar 
month)  to  travel  through  each  "  house  "  or  constellation.  In 
March  the  Sun  enters  the  constellation  known  by  the  Latin 
name  for  Fishes  (Pisces)  ;  in  June  it  gets  to  the  group  known 
as  the  Twins  (Gemini) ;  in  September  it  reaches  the  Virgin 
( Virgo) ;  in  December  it  is  with  the  Archer  (Sagittarius); 
and  the  following  March  it  enters  once  more  the  constellation 
of  Pisces.1 

1  The  Sun  is  commonly  said  to  be  at  the  "First  Point  of  Aries"  (the  Ram) 
at  the  Spring  Equinox.  This  is  true  only  at  certain  long  distant  intervals,  as 
will  be  explained  in  Chapter  XII.  The  "point"  has  reference  to  the  Earth's 
orbit,  and  not  to  the  stars.  It  was  named  after  the  constellation  which  happened 
at  the  time  to  be  beyond  the  Sun  in  March. 


APPARENT  MOTIONS   OF   HEAVENLY  BODIES     11 

LUNAR  MUSIC 

The  positions  and  motions  of  the  Moon  are  about  the  same 
as  those  of  the  Sun,  only  the  Moon  hangs  back  more  and  loses 
a  revolution  in  a  lunar  "  moonth,"  or  month,  instead  of  losing 
one  in  a  year.  In  its  backward  drift  it  therefore  catches  up 
with  the  Sun  nearly  thirteen  times  in  a  solar  year. 

Eclipses.  —  The  various  phases  of  the  Moon  show  that  it  is 
a  dark  body,  like  our  Earth,  lighted  up  on  one  side  by  the  Sun. 
They  also  show  that  it  is  nearer  to  us  than  the  Sun.  Some- 
times, indeed,  it  passes  exactly  between  us  and  the  Sun,  pro- 
ducing what  is  known  as  an  Eclipse  of  the  Sun.  When  it  is 
opposite  to  the  Sun,  the  shadow  of  the  Earth  sometimes  falls 
on  it,  producing  what  is  known  as  an  Eclipse  of  the  Moon.  The 
reason  why  there  is  not  an  eclipse  at  every  "  conjunction  "  and 
"  opposition  "  of  the  Sun  and  Moon  is  that  the  path  of  the  lat- 
ter, although  nearly  on  the  same  plane  as  that  of  the  Sun,  does 
not  exactly  coincide  with  it.  The  two  paths,  therefore,  appear 
to  cross  or  intersect,  in  the  same  way  that  the  Ecliptic  and  the 
Equator  cross  each  other. 

PLANETARY  MUSIC 

The  larger  planets  all  keep  on  or  near  the  Sun's  path,  but 
their  apparent  motions  are  more  irregular,  and  each  has  a  period 
of  its  own,  varying  from  a  few  months  to  many  generations. 

Those  known  as  Mercury  and  Venus  appear  to  drift  back- 
ward and  forward  on  each  side  of  the  Sun.  They  never  go 
very  far  from  it,  and  are  therefore  seen  only  shortly  before  sun- 
rise or  soon  after  sunset. 

The  other  planets  appear  to  drift  eastward  among  the  stars 
that  lie  along  the  path  of  the  Sun  and  Moon.  But  when  they 
get  nearly  opposite  to  the  Sun  (that  is,  when  they  pass  the 
south  about  midnight)  this  eastward  drift  is  reversed  for  a  time, 
so  that  each  planet  appears  to  make  a  loop  in  the  star-sphere. 
But  they  never  go  far  away  from  the  Ecliptic,  or  path  of  the 
Sun  (see  Figure  97). 


12     HOW  TO   KNOW   THE   STARRY   HEAVENS 

North  and  South  of  the  Equator.  —  If  our  observer  takes  his 
apparatus  north  or  south  of  the  Equator,  and  tips  it  up  as 
before,  when  observing  the  stars,  he  will  find  that  the  positions 
and  motions  of  Sun,  Moon,  and  planets  can  all  be  approximately 
marked  out  on  the  hoop  that  represents  the  Ecliptic.  This 
will  be  true  for  any  and  every  part  of  the  Earth's  surface.  The 
Moon  and  the  large  planets  are  never  found  in  any  other  part 
of  the  sky  than  on  (or  close  to)  the  Sun's  path,  or  Ecliptic.  The 
same  apparatus  will  show  why  the  days  are  long  in  June  north 
of  the  Equator,  and  long  in  December  to  the  south  of  that  line. 

Going  East  and  West.  —  So  far  the  observer  has  travelled 
only  north  and  south.  If  he  travels  to  the  east  or  west,  he  will 
find  that  no  change  is  needed  in  his  apparatus  so  long  as  he 
does  not  change  his  latitude. 

On  the  Equator,  for  example,  the  motions  of  the  heavenly 
bodies  are  the  same  whether  the  observer  is  in  Africa,  the  East 
Indies,  or  in  America.  The  only  difference  is  in  time.  If  he 
could  telegraph  from  equatorial  Africa  at  midnight,  and  get 
immediate  answers  from  the  East  Indies  and  America,  he  would 
find  that  it  was  already  sunrise  in  the  East  Indies,  whilst  it 
was  only  sunset  in  equatorial  America.  With  this  exception 
the  phenomena  observed  are  alike  on  all  parts  of  the  Earth 
lying  under  the  Equator.  The  same  is  true  of  any  other 
latitude. 

Although  our  apparatus  represents  very  fairly  the  angular 
distances  and  apparent  motions  of  the  heavenly  bodies,  it  does 
not  directly  throw  any  light  on  their  actual  distances  or  real 
motions. 

A  PRIMITIVE  EQUATORIAL 

It  will  be  well  for  all  who  have  not  made  a  study  of  the 
above  phenomena  to  observe  for  themselves  as  many  of  these 
apparent  motions  as  can  be  seen  from  the  part  of  the  world 
they  may  happen  to  live  in.  All  the  apparatus  that  is  really 
necessary  is  a  straight  stick  set  firmly  in  the  ground  (or  other- 
wise supported)  at  such  an  angle  that  it  will  point  to  the  Pole 


APPARENT   MOTIONS   OF   HEAVENLY   BODIES     13 

Star,  and  a  tube  (or  telescope)  attached  to  it  so  that  it  can  be 
moved  in  any  direction.  The  tube  or  telescope  can  then  be 
rotated  so  as  to  follow  the  diurnal  motion  of  any  of  the  heav- 
enly bodies.  When  the  tube  is  at  right  angles  to  its  support 
it  is  pointing  to  the  celestial  Equator  (see  Figure  6).  Rude 


Fio.  6.  —  AN  ADJUSTABLE  EQUATORIAL,  SUITABLE  FOR 
ANY  PART  OF  THE  WORLD 

The  axis  is  clamped  in  such  a  position  that  its  ends  point  to  the 
poles  of  the  heavens. 

as  this  method  of  observation  may  seem,  it  is  capable  of  lead- 
ing intelligent  observers  to  a  correct  solution  of  the  main  prob- 
lems of  astronomy. 

A  very  interesting  method  of  observing  the  daily  motions  of 
the  stars  is  to  point  a  camera  to  some  part  of  the  sky  on  a  clear 
starlight  night,  and  leave  the  plate  exposed  for  an  hour  or  so. 
On  developing  the  print  it  will  be  found  that  each  star  has  left 
a  trail  on  the  plate.  Figure  7  is  a  photograph  of  the  stars  sur- 


14     HOW   TO   KNOW   THE   STARRY   HEAVENS 

rounding  the  North  Pole.  The  further  the  star  is  from  the 
centre  of  rotation,  the  longer  and  straighter  is  the  trail  it  makes 
on  the  plate. 

Figure  8  shows  the  constellation  of  the  Great  Bear  (or  the 
Dipper,  as  it  is  often  called),  repeated  four  times,  to  show  its 
position  in  the  northern  skies  every  six  hours.  It  will  be  seen 
that  the  two  "  Pointers  "  are  always  in  a  straight  line  with  the 
star  which  happens  to  be  near  the  axis  of  rotation.  This  star 
is  commonly  known  as  (Stella)  Polaris,  or  the  Pole  Star. 


FIG.   7.  —  NORTHERN   STAR-TRAILS 
Photographed  by  Barnard,  with  twelve  hours'  exposure. 


FIG.   8.  —  THE   DIPPKR,   OR  GREAT   BEAR,   AT  INTERVALS  OF   Six   HOURS 
It  will  be  seen  that  the  two  "pointers"  are  always  in  a  line  with  the  Pole-star. 


\ 


CHAPTER  II 

KIVAL  THEORIES  TO  EXPLAIN  THE  APPARENT   MOTIONS 
OF  THE  HEAVENLY  BODIES 

(A)     THE  EARTH-CENTRED   THEORIES  OF  THE  UNIVERSE 

"Then  the  Evening  (Erev)  and  the  Morning  ( Voker)  brought  to  a  close  the 
Third  Day  ( Yom}. 

"And  the  Mighty  Ones  (Elohim)  said  :  '  Let  there  be  luminaries  in  the  Ham- 
mered Plate  (Rakia)  of  the  sky,  to  separate  the  Day  (Yom)  from  the  Night 
(Lylah};1  let  them  be  for  signs  and  to  mark  the  seasons,  Days  (Yamim),  and 
years  ;  let  them  serve  as  luminaries,  in  the  Hammered  Plate  of  the  sky,  to  give 
light  upon  the  Earth.'  And  it  was  so. 

"  And  the  Mighty  Ones  made  two  great  luminaries,  the  larger  one  to  preside 
over  the  Day  ( Yom),  and  the  smaller  one  over  the  Night  (Lylah).  [They  made] 
the  stars  also. 

"  The  Mighty  Ones  placed  them  in  the  Hammered  Plate  of  the  sky,  to  give  light 
upon  the  Earth,  to  preside  over  the  Day  (Yarn),  and  the  Night  (Lylah},  and  to 
separate  the  light  from  the  darkness.  The  Mighty  Ones  saw  that  it  was  good. 

"Then  the  Evening  (Erev}  and  the  Morning  (Voker}  brought  to  a  close  the 
Fourth  Day  (Yom}."  —  Book  of  Origins,  I,  13-19  (A.  ZazeVs  Translation). 

EARLY  FLAT-WORLD  SUPPOSITIONS 

IN  trying  to  explain  the  observed  motions  real  (or  apparent) 
of  the  heavenly  bodies  the  ancients  were  handicapped  by 
their  ignorance  of  the  world  itself.  This  appeared,  from  their 
local  standpoint,  to  be  a  flat  though  uneven  surface,  the  lower 
parts  of  which  were  filled  with  water.  Their  experiences  on 
this  Earth  also  prevented  them  from  realising  the  possibility  of 
anything  solid  and  heavy  remaining  suspended  in  space  without 
falling  anywhere.  Their  entire  ignorance  as  to  the  nature, 
dimensions,  and  distances  of  the  celestial  bodies  led  them  to 

1  Lylah  was  personified  by  the  Israelites  as  Lilith,  the  first  wife  of  Adam. 
Isaiah  xxxiv,  14  (R.  V.  — Margin). 


16     HOW  TO   KNOW   THE   STARRY   HEAVENS 

suppose  that  they  were  put  in  the  heavens  by  somebody  to  throw 
light  on  the  Earth,  or  to  relieve  the  monotony  of  the  sky.  With 
them  the  Earth  itself  was  the  Universe,  and  even  those  who 
recognised  the  importance  of  some  of  the  most  prominent  celes- 
tial objects  made  the  natural  mistake  of  supposing  them  to  be 
Gods  who  ruled  the  Earth  from  their  thrones  on  high. 

UNDERLYING  FACTS 

Yet  even  three  and  four  thousand  years  agone  there  were 
individuals  who  had  discovered  that  "  things  are  not  what  they 
seem."  Some  of  the  real  facts  relating  to  the  Universe  were 
known  to  a  few  learned  men  among  the  ancient  Babylonians, 
Egyptians,  Chinese,  Greeks,  and  Hindus.  But  the  world  was 
not  ready  for  their  teachings,  and  during  the  Dark  Ages  that 
followed  the  establishment  of  Christianity  the  few  truths  that 
were  known  were  trampled  under  foot,  like  pearls  cast  before 
swine. 

However,  trampled  pearls  are  apt  to  come  to  light  again. 
Facts  are  stubborn  things,  and  will  not  permanently  down. 
So  the  lost  facts  have  been  rediscovered  in  modern  times,  and 
largely  supplemented  by  fresh  ones. 

Let  us  glance  briefly  at  some  of  the  primitive  ideas  held  by 
the  ancients  with  regard  to  the  Universe,  so  that  we  may  com- 
pare them  with  more  modern  explanations.  We  can  then  decide 
as  to  which  best  fit  the  observed  phenomena,  and  are,  on  that 
account,  the  most  deserving  of  credence. 

THE  CANOPY  THEORY 

The  world  we  live  in  was  at  first  supposed  to  be  flat,  or  nearly 
so,  with  a  massive  firmament  resting  on  the  mountains  at  the 
edge  and  spanning  the  whole  Earth. 

To  the  ancient  Egyptians  the  sky  was  the  bosom  of  Neit,  a 
celestial  ocean  across  which  the  divine  Sun,  Moon,  and  planets 
were  carried  in  boats.  In  Greece  it  was  supposed  to  be  a  solid 
canopy,  across  one  part  of  which  Helios,  the  Sun-God,  daily 


RIVAL   THEORIES  17 

drove  in  a  chariot  of  gold,  while  his  sister  Selene,  the  Moon- 
Goddess,  followed  him  in  a  chariot  of  silver.  Mount  Olympus 
was  supposed  to  reach  up  to  the  highest  part  of  this  canopy. 
On  the  summit  of  this  holy  mountain  was  the  palace  of  Zeus, 
king  of  all  the  Gods.  There  the  Greater  Deities  abode,  ruling 
the  world  below  to  suit  themselves,  and  dealing  out  a  very  pecu- 
liar kind  of  justice  to  the  unfortunate  mortals  who  lived  thereon. 

Ancient  books,  as  a  rule,  did  not  discuss  or  assert  these  things, 
any  more  than  modern  books  discuss  or  assert  the  conclusions 
of  modern  astronomy.  They  merely  alluded  to  them,  taking  them 
for  granted  as  well-ascertained  facts  which  were  useful  for  illus- 
tration, but  which  it  would  be  folly  to  argue  about  or  assert. 
Thus  one  of  the  characters  in  the  Hebrew  drama  of  Job  casu- 
ally mentioned  that  this  firmament  was  "  spread  out "  (Job  ix,  8) 
"as  strong  as  a  molten  mirror"  (Job  xxxvii,  18  —  R.  V.).  In 
the  same  way  the  Mohammedan  Koran  sought  to  show  the  fine 
workmanship  of  Allah  by  pointing  out  that  he  had  stretched  the 
firmament  across  the  entire  world  without  a  crack  in  it. 

The  Hebrew  word  for  firmament  (Ralda)  really  means  a  ham- 
mered plate  of  metal  (Ex.  xxxix,  3),  and  all  its  Greek  and  Latin 
equivalents  have  afirm  or  solid  meaning.  The  modern  idea  that 
the  writers  meant  an  expanse  is  seen  to  be  absurd  when  we  notice 
that  it  was  created  (Gen.  i,  1),  or  made  (Gen.  i,  7) ;  that  it  was 
spread  out  over  the  Earth  (Job  ix,  8) ;  and  that  it  had  windows 
in  it  (Gen.  vii,  11)  ;  also  that  the  Tower  of  Babel  was  intended 
to  reach  up  to  it  (Gen.  xi,  4) ;  and  that  the  top  of  Jacob's  ladder 
rested  against  it  (Gen.  xxviii,  12).  The  mountains  which  were 
supposed  to  support  this  "  hammered  plate  of  heaven  "  were  natu- 
rally spoken  of  as  the  pillars  of  heaven  (Job  xxvi,  11). 

When  the  writer  of  the  Apocalypse  was  describing  the  ap- 
proaching end  of  the  world  he  made  an  earthquake  shake  the 
stars  out  of  this  firmament  on  to  the  ground  "  as  a  fig-tree  casteth 
her  unripe  figs  when  she  is  shaken  of  a  great  wind."  He  ended 
by  letting  the  heavens  roll  together,  as  a  scroll  does  when  the 
ends  are  released  (Rev.  vi,  13-14  —  R.  V.). 

The  Venerable  Bede,  an  eminent  Christian  writer  of  the 

2 


18     HOW   TO   KNOW   THE   STARRY   HEAVENS 

seventh  century,  considered  the  Earth  to  be  flat  (or  perhaps 
convex),  with  a  star-spangled  canopy  over  it.  This  canopy  he 
supposed  to  be  like  an  umbrella,  with  its  centre  at  the  Pole 
Star.  The  daily  motion  of  the  heavenly  bodies  he  explained 
by  supposing  the  canopy  to  spin  round,  like  the  tent  over  the 
"  merry-go-rounds  "  of  our  country  fairs.  His  ideas  on  the  sub- 
ject are  a  curious  mixture  of  accurate  observation  and  childlike 
speculation.  He  says : 

"  The  Creation  was  accomplished  in  six  days.  The  Earth  is  its 
centre  and  its  primary  object.  The  Heaven  is  of  a  fiery  and  subtile 
nature,  round  and  equidistant  from  every  part,  as  a  canopy  from  the 
centre  of  the  Earth.  It  turns  round  every. day  with  ineffable  rapidity, 
only  moderated  by  the  resistance  of  the  seven  planets,  three  above 
the  Sun  —  Saturn,  Jupiter,  Mars  —  then  the  Sun  ;  three  below  — 
Venus,  Mercury,  the  Moon.  The  stars  go  round  in  their  fixed  courses, 
the  northern  perform  the  shortest  circle.  The  Highest  Heaven  .  .  . 
contains  the  angelic  virtues.  .  .  .  The  Inferior  Heaven  is  called  the 
Firmament,  because  it  separates  the  superincumbent  waters  from  the 
waters  below." 

THE  CRYSTAL  SPHERES 

Such  were  the  primitive  ideas  of  unenlightened  men  with  regard 
to  the  Universe.  Sometimes,  however,  the  problem  was  investi- 
gated in  a  scientific  spirit.  It  was  then  readily  seen  that  the 
celestial  phenomena  could  not  be  explained  on  the  canopy 
theory. 

Observation,  as  well  as  theory,  ultimately  led  to  the  overthrow 
of  this  primitive  idea  of  a  solid  star-strewn  firmament  resting 
on  the  mountains.  For  many  of  these  so-called  "pillars  of 
heaven "  had  been  ascended,  and  no  "  hammered  plate  of 
heaven"  had  been  found  resting  on  them. 

So  new  theories  arose,  each  of  which  came  a  little  nearer  the 
truth  than  the  one  before.  It  was  suggested  that  the  world  was 
inside  a  crystal  globe  or  sphere,  to  which  the  stars  were  attached. 
The  nightly  motions  of  the  stars  were  explained  by  supposing 
that  this  crystal  sphere  rolled  over  every  twenty-four  hours. 


RIVAL   THEORIES 


19 


This  theory  explained  very  well  the  motions  of  the  stars,  but 
did  not  fit  in  with  the  more  varied  movements  of  the  Sun,  Moon 
and  five  planets.  Some  explained  their  irregularities  of  motion 
by  supposing  that  they  were  carried  around  with  the  stare,  but 
that,  instead  of  being  fixed  to  the  revolving  sphere,  like  the  stars, 
they  were  at  liberty  to  crawl  around  on  it  very  slowly,  like  so 


FIG.  9.  —  THE  PTOLEMAIC  SYSTEM 

many  insects.  Others  suggested  that  each  of  these  seven  wan- 
derers had  a  crystal  sphere  all  to  himself  (see  II  Cor.  xii,  2). 
The  seven  spheres  were  supposed  to  be  one  inside  the  other. 
Each  was  thought  to  share  in  the  general  daily  rotation,  but  to 
lag  behind  or  have  a  slight  independent  motion  of  its  own  (see 
Figure  9). 

Even  this  far-fetched  notion  did  not  fit  in  satisfactorily  with 
the  observed  phenomena.  So  Tycho  Brahe*  suggested  that  the 
Earth  was  a  globe,  spinning  round  on  its  axis  every  twenty-four 


20      HOW   TO   KNOW   THE    STARRY   HEAVENS 

hours ;  that  the  Sun  and  Moon  went  around  the  Earth  in  a  year 
and  in  a  month,  respectively ;  and  that  the  five  planets  went 
around  the  circling  Sun  (see  Figure  10). 


FIG.  10.  —  THE  TYCHONIC  SYSTEM 
EPICYCLES 

As  even  this  complex  arrangement  did  not  fit  in  with  the 
observed  motions,  the  planets  were  then  supposed  to  move  in 
a  series  of  eccentrics  around  their  ideal  orbits,  with  the  star- 
sphere  outside  of  all.  For  a  time  this  theory  was  thought  to 
explain  the  observed  motions.  But  it  was  such  a  complex, 
improbable,  lumbering,  incomprehensible,  and  absurd  theory 
that  Alphonso,  king  of  Castile,  ventured  the  remark  that  if  he 
had  been  consulted  by  the  Creator  he  could  have  considerably 
improved  upon  the  plan. 


RIVAL   THEORIES  21 


FAUSTUS  ON  THE  SPHERES 

These  mediaeval  speculations  are  well  illustrated  by  the  fol- 
lowing dialogue  from  Marlowe's  "  Faustus"  written  toward  the 
close  of  the  sixteenth  century. 

"  Faust.  Tell  me,  are  there  many  heavens  above  the  Moon  ? 

Are  all  celestial  bodies  but  one  globe, 

As  is  the  substance  of  this  centric  Earth  ? 

Mephistopheles.  As  are  the  elements,  such  are  the  spheres, 

Mutually  folded  in  each  other's  orb. 

And,  Faustus, 

All  jointly  move  upon  one  axletree, 

Whose  terminine  is  termed  the  World's  wide  pole : 

Nor  are  the  names  of  Saturn,  Mars,  or  Jupiter 

Feign'd,  but  are  erring  stars. 

Faust.  But,  tell  me,  have  they  all  one  motion,  both  situ  et  tempore  ? 

MepJi.  All  jointly  move  from  east  to  west  in  twenty-four  hours  upon 
the  poles  of  the  World,  but  differ  in  their  motion  upon 
the  poles  of  the  zodiac. 

Faust.  Tush,  these  slender  trifles  Wagner  can  decide : 

Hath  Mephistopheles  no  greater  skill? 

Who  knows  not  the  double  motion  of  the  planets? 

The  first  is  finished  in  a  natural  day; 

The  second  thus;  as  Saturn  in  thirty  years,  Jupiter  in  twelve; 
Mars  in  four ;  the  Sun,  Venus,  and  Mercury  in  a  year ; 
the  Moon  in  twenty-eight  days.  Tush,  these  are  fresh- 
men's suppositions.  But,  tell  me,  hath  every  sphere  a 
dominion  or  intellitjencia  ? 

Meph.  Aye. 

Faust.  How  many  heavens  or  spheres  are  there  ? 

Meph.  Nine ;  the  seven  planets,  the  firmament,  and  the  empyreal 
heaven. 

Faust.  Well,  resolve  me  in  this  question  ;  why  have  we  not  con- 
junctions, oppositions,  aspects,  eclipses,  all  at  one  time, 
but  in  some  years  we  have  more,  in  some  less  ? 

Meph.  Per  incequalem  motum  respectu  totius. 

Faust.  Well,  I  am  answered." 


22      HOW   TO   KNOW   THE   STARRY   HEAVENS 

The  writer  of  a  recent  magazine  article l  sums  up  these  old 
ideas  of  the  Universe  very  neatly.  He  says : 

"  To  the  men  of  the  Middle  Ages  the  world  was  a  little  space  shut 
tight  within  a  wheelwork  of  revolving  spheres.  It  was  compendious,, 
complete,  ingenious,  like  a  toy  in  a  crystal  box.  Beyond  the  outer 
shell  nothing  existed.  The  heavens  were  uncorruptible.  No  change 
could  occur  in  the  whole  system,  save  in  the  Earth  alone.  The  Uni- 
verse was  created  for  the  sole  use  of  man.  It  was  small  and  finite." 

THE  ROUND  WORLD 

While  all  these  speculations  were  going  on,  people  had  been 
going  to  and  fro  on  the  Earth,  and  travelling  up  and  down  on 
it.  In  this  way  they  had  discovered  for  an  actual  fact  that  the 
world  is  not  flat,  but  is  a  round  ball,  8,000  miles  thick,  suspended 
in  space,  with  the  starry  heavens  on  every  side  of  it. 

This  being  the  case,  it  follows  that  if  the  stars  are  fixed  to  a 
massive  firmament,  it  is  not  a  mere  "  dish-cover  "  or  umbrella 
over  a  flat  Earth,  but  is  in  the  form  of  a  hollow  crystal  sphere, 
rolling  over  (to  the  west)  every  twenty-four  hours,  with  the  round 
World  in  the  centre,  supporting  itself  on  nothing. 

(B)    THE  SUN-CENTRED   THEORY  OF  COPERNICUS. 

"The  first  formal  assertion  of  the  heliocentric  theory  was  made  in 
a  timid  manner,  strikingly  illustrative. of  the  expected  opposition.  It 
was  by  Copernicus,  a  Prussian,  speaking  of  the  revolutions  of  the 
heavenly  bodies  ;  the  year  was  about  1536.  In  his  preface  ...  he 
complains  of  the  imperfections  of  the  existing  system,  states  that  he 
has  sought  among  ancient  writers  for  a  better  way,  and  so  had  learned 
the  heliocentric  theory.  ...  In  their  decree  prohibiting  [the  work  of 
Copernicus],  '  De  RevolutionilmsJ  the  Congregation  of  the  Index, 
March  5,  1616,  denounced  the  new  system  of  the  Universe  as  'that 
false  Pythagorean  doctrine  utterly  contrary  to  the  Holy  Scriptures.' 
.  .  .  The  opinions  thus  defended  by  the  Inquisition  are  now  objects 
of  derision  to  the  whole  civilized  world." 2 

1  Dr.  E.  S.  Holden,  in  the  "  Popular  Science  Monthly,"  November,  1903. 

2  Dr.  J.  W.  Draper. 


RIVAL  THEORIES  23 

"  People  gave  heed  to  an  upstart  astrologer  who  strove  to  show  that 
the  Earth  revolves,  not  the  heavens  or  the  firmament,  the  Sun  and 
Moon.  .  .  .  This  fool  wishes  to  reverse  the  entire  science  of  astronomy." l 

The  final  outcome  of  all  these  speculations  was  that  the  whole 
of  the  Earth-centred  theories  were  thrown  overboard,  and  re- 
placed by  an  old  Sun-centred  theory  originally  brought  from 
India  by  Pythagoras.  According  to  this  new  yet  ancient  theory, 
the  stars  are  practically  immovable  bodies  suspended  far  off  in 
space,  and  the  Sun  is  the  centre  around  which  all  the  planets, 
including  the  Earth  itself,  revolve  (see  Figure  11). 


FIG.  11.  —  COPERNICAN  SYSTEM 
Including  four  of  the  most  tilted  orbits  of  Minor  Planets.     Neptune's  orbit  is  here  omitted. 

It  has  been  found,  however,  that  the  Moon  does  actually  go 
around  the  Earth,  completing  a  revolution  in  about  a  month. 
The  daily  motion  of  all  the  heavenly  bodies  is  not  real,  but  only 
apparent.  It  is  explained  by  the  fact  that  the  Earth  itself  rolls 
completely  over  (to  the  east)  every  twenty-four  hours,  at  the 
same  time  that  it  travels  around  the  Sun  once  every  year.  This 
daily  rotation  of  the  Earth  causes  all  the  heavenly  bodies  to 
appear  to  turn  in  the  opposite  direction. 

The  peculiarities  of  the  Sun's  annual  drift  to  the  north  and 
south,  and  the  resulting  seasons,  are  readily  explained  by  the 

l  Martin  Luther. 


24     HOW  TO   KNOW  THE   STARRY   HEAVENS 

fact  that  the  Earth  has  a.  heavy  "  list "  to  one  side,  with  reference 
to  its  path  around  the  Sun  (see  Figure  12).  The  other  planets 
are  also  drifting  around  the  Sun,  in  the  same  general  direction, 
but  at  different  distances  from  it. 

This  Indian  system  of  cosmogony  is  now  known  as  the  Coper- 
nican  Theory,  because  Copernicus  first  established  its  truth  in 
modern  Europe.  It  explains  the  motions  of  the  heavenly  bodies 
so  well  that  there  is  no  doubt  about  its  being  true  as  far  as  it 


FIG.  12.  —  RELATIVE  POSITIONS  OF  EARTH  AND  SUN  AT  THE  FOUR  SEASONS 

goes.  In  spite  of  the  long-continued  opposition  of  unprogres- 
sive  theologians,  it  has  now  been  adopted  by  all  competent 
judges,  and  is  accepted,  on  hearsay,  even  by  those  who  do  not 
realise  the  subordinate  position  to  which  it  reduces  our  Earth, 
and  by  those  who  do  not  profess  to  be  competent  to  judge  as  to 
its  correctness. 

The  adoption  of  this  theory  has  led  to  the  solution  of  a  mul- 
titude of  otherwise  inexplicable  phenomena.  Without  it,  the 
planetary  bodies  appeared  to  be  swinging  around  us  in  a  labyrinth 
of  perplexing  knots  and  meaningless  tangles.  As  a  result  of  its 
adoption,  the  knots  and  tangles  have  all  been  unravelled,  and 
the  structure  and  dimensions  of  the  Solar  System  have  been 


RIVAL  THEORIES  25 

tolerably  well  ascertained.  The  telescope  and  other  opitical  in- 
struments have  now  greatly  increased  our  knowledge  of  the 
heavenly  bodies  generally,  and  have  revealed  to  us  similar  sys- 
tems moving  in  actual  conformity  with  the  Copernican  Theory. 

This  same  theory  has  also  enabled  astronomers  to  apply  them- 
selves, not  entirely  without  success,  to  the  task  of  ascertaining 
the  structure,  and  measuring  the  distances  and  dimensions,  of 
the  more  distant  luminaries  known  to  us  by  the  misleading  name 
of  "  fixed  stars." 

Every  observed  peculiarity  is  explained  by  this  theory,  without 
any  absurd  and  impossible  suppositions  like  the  "  eccentrics " 
and  "  epicycles  "  of  other  theories.  And  many  facts  have  been 
discovered  by  following  it  up  to  its  logical  conclusions.  It  is 
therefore  the  true  explanation  of  the  mechanism  of  the  Universe. 

How  and  why  these  movements  of  the  heavenly  bodies  are 
kept  up  will  be  briefly  dealt  with  in  subsequent  chapters. 


CHAPTER  III 

PRINCIPLES   UTILISED  FOR   MEASURING  THE   UNIVERSE 

"And  there  was  given  me  a  reed  like  unto  a  rod,  and  the  angel  stood,  saying, 
Rise  and  measure  the  temple  of  God."  —  Rev.  xi,  1. 

"And  he  that  talked  with  me  had  a  golden  reed  to  measure  the  city,  .  .  . 
and  he  measured  the  city  with  the  reed,  twelve  thousand  furlongs.  The  length 
and  the  breadth  and  the  height  of  it  are  equal."  —  Rev.  xxi,  15,  16. 

"The  measure  of  the  Moon's  distance  involves  no  principle  more  abstruse  than 
the  measure  of  the  distance  of  a  tree  on  the  opposite  side  of  a  river."  —  Sir  George 
Airy. 

HOW  IT  IS  DONE 

I  WILL  now  say  a  few  words  about  the  way  in  which  as- 
tronomers have  been  enabled  to  find  out  the  distances  and 
dimensions  of  many  of  the  objects  which  -compose  the  LTni verse. 
It  was  very  early  recognised  that  the  heavenly  bodies  are  not 
all  at  the  same  distance  from  us. 

STARS  ARE  BEYOND  PLANETS 

The  stars,  for  example,  have  a  far-away  look  and  a  fixity  of 
position  that  would  naturally  lead  one  to  think  that  they  were 
beyond  the  larger,  brighter,  and  more  active  luminaries  which 
are  found  on  or  near  the  Ecliptic.  This  was  proved  beyond  a 
doubt  when  observers  at  a  distance  from  one  another  compared 
notes.  For  it  was  sometimes  found  that  when  an  observer  in 
the  north  saw  a  certain  planet  a  little  to  the  south  of  a  particu- 
lar star,  an  observer  in  the  south  would  see  it  north  of  the  same 
star.  The  only  possible  explanation  of  this  is  that  the  planet  is 
nearer  to  us  than  the  stars. 

THE   ORDER  OF  THE  PLANETS 

Leaving  the  "  fixed "  stars  out,  there  were  seven  celestial 
"  wanderers  "  known  to  the  ancients.  Of  these,  two  appear  to 


PRINCIPLES   FOR   MEASURING   THE   UNIVERSE    27 

be  very  much  nearer  to  us  than  the  other  five.  The  Sun,  for 
example,  is  evidently  either  very  near  or  very  large,  bright,  and 
hot.  But  the  Moon  is  nearer  to  us  than  the  Sun,  for  it  some- 
times passes  in  front  and  shuts  off  its  light  and  heat  from  us. 
As  it  also  passes  between  us  and  every  other  celestial  object 
that  comes  in  its  way,  it  is  evidently  the  nearest  of  all  the 
heavenly  bodies. 

Now  the  Moon  performs  its  circuit  of  the  heavens  in  less  time 
than  any  other  wanderer.  It  seems  natural,  then,  to  suppose 
that  the  wanderers  which  take  the  most  time  to  perform  their 
circuit  are  the  farthest  away  from  their  common  centre  of 
revolution. 

This  reasoning  led  the  early  astronomers  to  regard  slow-moving 
Saturn  as  the  most  distant  planet.  The  stately  Jupiter  they 
put  next,  followed  by  fiery  Mars.  As  regards  the  other  three, 
there  was  some  difference  of  opinion,  due  to  the  fact  that,  on  the 
Earth-centre  theory,  their  real  motions  were  not  distinguished 
from  their  apparent  ones,  due  to  perspective. 

But  when  once  it  was  recognized  that  the  Sun  was  the  centre 
around  which  the  planets  turned,  it  became  evident  that  our  own 
populous  Earth  and  pale-faced  Moon  were  travelling  in  partner- 
ship, next  to  Mars ;  that  "  Venus  the  beautiful "  followed ;  and 
that  fast-flying  Mercury  kept  nearest  to  the  central  Sun. 

COMPARATIVE  DISTANCES 

The  order  of  the  planets  being  thus  settled,  the  next  thing 
was  to  ascertain  their  distances  from  the  Sun. 

In  the  case  of  the  inferior  or  inner  planets,  Mercury  and 
Venus,  their  proportional  distances  from  the  Sun  were  easily 
found.  All  that  had  to  be  done  was  to  point  one  leg  of  a  pair 
of  dividers,  or  compasses,  at  the  setting  or  rising  Sun,  and  the 
other  leg  at  the  planet  Venus  when  at  its  greatest  angular  dis- 
tance from  it,  as  an  Evening  or  Morning  Star.  The  dividers 
were  then  laid  on  a  sheet  of  paper,  and  two  lines  drawn 
to  indicate  the  V  shape  of  the  open  dividers  (see  S  E  V  in 
Figure  13). 


28     HOW   TO   KNOW   THE   STARRY   HEAVENS 


The  Earth  was  then  supposed  to  be  at  E,  where  the  two  lines 
come  together,  and  the  Sun  was  supposed  to  be  at  the  other  end 
(S)  of  one  of  the  lines.  Venus  would  evidently  be  somewhere 
on  the  line  E  V. 

Taking  it  for  granted  that  the  planetary  orbits  were  circular, 
a  circle  was  then  drawn  through  E  from  S  as  a  centre.  This 

represented  the  Earth's  or- 
bit. Another  and  smaller 
circle  was  drawn  from  the 
same  centre,  just  large 
enough  to  touch  the  other 
arm,  E  V.  This  circle 
evidently  represented  the 
orbit  of  the  planet  Venus. 
The  same  process  was 
gone  through  with  the 
planet  Mercury,  and  the 
result  transferred  to  the 
same  figure  (see  S  E  M). 

On  measuring  the  radii 
or  semi-diameters  of  these 
three  circles,  representing 

the  planetary  orbits,  it  was  found  that  their  lengths  varied 
in  the  ratio  of  100,  72,  and  38.  These  figures,  therefore, 
represent  the  relative  distances  of  the  Earth,  Venus,  and 
Mercury. 

A  comparison  of  the  distances  with  the  times  of  revolution 
then  enabled  the  relative  distances  of  the  superior  or  outer 
planets  to  be  computed  by  means  of  their  times  of  revolution, 
taking  it  for  granted  that  they  all  obeyed  the  same  law,  what- 
ever that  law  might  be. 

The  result  was  that  the  distances  of  the  outer  planets,  when 
computed  on  the  same  scale  as  the  inner  ones  (=100  to  the 
Earth's  distance),  were  found  to  be  152,  520,  and  953. 


FIG.  13.  —  ORBITS  OF  MERCURY,  VENUS, 
AND  EARTH 


PRINCIPLES   FOR   MEASURING   THE   UNIVERSE    29 

PLANETS  MOVE  IN  ELLIPSES 

It  may  be  as  well  to  state  here,  that  while  the  above  observa- 
tions were  being  made  it  was  discovered  that  the  orbits  of  the 
inner  planets  are  not  exactly  circular,  but  slightly  egg-shaped, 
or,  rather,  elliptical.  It  has  since  been  found  that  the  paths  of 
all  the  planets  share  this  peculiarity,  the  cause  of  which  has 
also  been  ascertained. 

NEWLY  DISCOVERED  PLANETS 

Since  the  invention  of  the  telescope  two  large  planets  have 
been  discovered  beyond  the  orbit  of  Saturn.  They  bear  the 
names  of  Uranus  and  Neptune.  On  the  same  scale  as  that  used 
above,  their  distances  from  the  Sun  are  represented  by  the 
numbers  1,920  and  3,000. 

A  great  number  of  small  planets  have  also  been  discovered  in 
the  interval  between  the  orbits  of  Mars  and  Jupiter.  Their 
numbers  are  so  great,  their  sizes  so  small,  and  their  orbits  so 
peculiar,  that  astronomers  formerly  looked  upon  them  as  the 
scattered  fragments  of  larger  planets  which  had  met  with  an 
accident.1 

ACTUAL  DISTANCES 

The  comparative  distances  of  all  the  planets  having  been  thus 
discovered,  all  that  had  to  be  done  was  to  find  the  real  distance 
of  one  of  them  in  miles.  All  the  other  distances  could  then  be 
readily  computed  in  miles. 

It  took  many  generations  to  solve  this  little  problem,  and  even 
yet  the  answer  is  not  as  free  from  error  as  could  be  wished.  It 
has,  however,  been  solved,  with  a  very  fair  amount  of  accuracy, 
by  several  independent  methods.  The  distances  usually  meas- 
ured are  those  of  the  neighbouring  planets  when  they  are  at 
their  least  distances  from  us  or  are  otherwise  favourably  placed. 

1  The  orbits  of  four  of  these  "  Asteroids  "  are  shown  in  Figure  11.  It  will  be 
noticed  that  the  four  represented  do  not  lie  in  the  same  general  plane  as  those  of 
the  larger  planets,  but  are  more  or  less  tilted  up,  some  one  way,  and  some  another. 
These,  however,  are  exceptions.  The  majority  move  in  or  near  the  general  plane. 


30     HOW  TO   KNOW  THE   STARRY   HEAVENS 

There  are  many  people  who  do  not  put  much  faith  in  celestial 
measures.  They  cannot  see  any  possibility  of  obtaining  them, 
seeing  that  we  cannot  stretch  a  tape-line  from  one  flying  world 
to  another.  There  are,  however,  a  number  of  ways  in  which  in- 
accessible distances  may  be  accurately  measured.  For  example, 
if  you  wish  to  measure  the  height  of  a  tree  without  ascending 
it,  all  you  have  to  do,  if  the  ground  is  level,  is  to  put  a  stick 
upright  in  the  sunshine,  and  measure  the  length  of  its  shadow. 
If  a  three-foot  upright  makes  a  three-foot  shadow,  then  a  hun- 
dred-foot shadow  indicates  that  the  tree  which  casts  it  is  a 
hundred  feet  high.  And  if  the  Sun  is  so  low  down  that  the 
three-foot  stick  makes  a  six-foot  shadow,  then  a  two-hundred- 
foot  shadow  will  indicate  that  the  tree  which  casts  it  is  a  hundred 
feet  high. 

There  are  other  methods  which  are  just  as  simple,  though 
most  of  them  require  more  elaborate  apparatus.  A  little  study 
will  show  that  celestial  and  other  inaccessible  measurements 
may  be  as  accurate  as  any  made  with  the  help  of  a  chain  or 
tape-line.  Let  us  see  what  are  the  principles  involved  and 
methods  employed. 

ESTIMATING  DISTANCES 

If  you  close  one  eye  and  keep  your  head  still,  you  will  find 
that  with  one  eye  alone  you  will  be  unable  to  judge  as  to  the 
distance  from  you  of  the  object  you  are  looking  at.  The  only 
exception  to  this  is,  that  if  you  already  know  the  size  of  the 
object  you  can  estimate  its  distance  by  noticing  whether  it 
appears  to  be  large  or  small. 

To  be  able  to  estimate  your  distance  from  any  object,  you 
must  either  move  your  head  or  open  the  other  eye,  so  as  to 
get  another  picture  of  it  to  compare  with  the  image  already  ob- 
tained. Then  you  can  estimate  with  a  tolerable  amount  of 
accuracy  how  far  the  object  is  from  you  (see  Figure  14). 

The  two  eyes  form  the  extremity  of  a  three-inch  base-line, 
and  if  you  draw  an  imaginary  line  from  each  eye  to  the  point 
you  are  looking  at,  you  will  obtain  a  three-cornered  or  triangular 


PRINCIPLES  FOR  MEASURING  THE  UNIVERSE    31 


figure  of  known  dimensions.     That  is,  you  will  know  (approxi- 
mately) the  length  of  all  its  three  sides. 


LAND-SURVEYING 


The  surveyor,  when  he  wishes  to  find  the  width  of  a  river 
without  crossing  to  the  other  side,  measures  off  a  base-line  on 
his  own  side  of  the  stream.  Then,  by  noting  with  his  instru- 
ments the  position  of  an  object  on  the  other  side  of  the  river,  as 


his    base-line,    he 
river  is. 

either  calculate  the 
result  by  means  of 


FIG.  14.  —  ESTIMATING  DIS- 
TANCES WITH  THE  EYES 


seen  from  each  end  of 
can  tell  how  wide  the 

In  this  case  he  can 
distance,  or  get  the 
a  diagram  on  paper. 
If  he  wishes  to  do  the 
latter,  he  draws  a  line 
to  represent  his  base- 
line, and  from  each 
end  of  it  sets  off  a  line 
at  the  same  inclination 
or  angle  to  it  as  that  used  on  his  real  base-line.  The  place 
where  these  two  lines  cross  each  other  represents  the  position 
of  the  object  observed  on  the  other  side  of  the  river  (see  Figure 
15).  By  measuring  the  sides  of  his  triangle  he  gets  the  dis- 
tance required  in  terms  of  his  base.  For  instance,  if  the  sides 
of  his  triangle  are  10  times  as  long  as  the  base  thereof,  and 
the  latter  is  10  yards  long,  then  the  width  of  the  river  is  100 
yards.1 

A  whole  continent  can  be  surveyed  in  the  same  way,  by 
measuring  off  three-cornered  areas  of  land,  and  using  every  dis- 
tance obtained  as  a  base  to  measure  other  distances  with.  In 
this  way  (with  certain  details  and  precautions  which  need  not 
be  here  specified)  the  shape  and  size  of  the  Earth  can  be 
obtained. 

1  A  right-angled  triangle  gives  the  best  results.  Those  who  wish  for  further 
details  will  find  them  in  the  next  chapter,  which  is  written  for  those  who  are  not 
afraid  of  a  little  simplified  trigonometry  and  diluted  mathematics. 


HOW  TO   KNOW  THE   STARRY   HEAVENS 


SKY-SURVEYING 

The  astronomer  then  finds  out  the  distance  of  the  Moon  in 
the  same  way,  by  using  a  measured  base-line  about  4,000  miles 
long.  As  he  cannot  see  one  end  of  his  base-line  from  the  other 
end  of  it,  he  gets  his  angles  indirectly,  by  polar  distances,  or  by 
observing  how  much  the  Moon  is  displaced  among  the  stars 
when  viewed  from  different  parts  of  the  world  at  the  same  time. 
The  same  principle,  rather  differently 
applied,  enables  him  to  tell  the  distance 
of  the  Sun. 

With  a  4,000-mile  base-line  the 
Moon's  distance  is  found  to  be  about 
60  times  as  long  as  the  base-line.  On 
multiplying  4,000  by  60  we  get  the 
Moon's  distance,  240,000  miles.1 

With  the  same  base-line  of  4,000 
miles  the  Sun's  distance  is  found  to  be 
about  388  times  greater  than  that  of  the 
Moon.  It  will  be  seen  that  the  longest 
base-line  we  can  get  is  very  short  when 
compared  with  the  distance  to  be  meas- 
ured ;  but  as  it  is  the  longest  available, 
astronomers  have  to  make  up  for  its 

shortness  by  using  different  methods  and  taking  advantage  of 
every  favourable  opportunity  to  correct  their  measurements. 
Now  388  times  240,000  comes  to  about  93,000,000  miles,  which 
is  approximately  the  Earth's  distance  from  the  Sun. 

As  we  already  know  the  comparative  distances  of  the  other 
planets  from  the  Sun,  their  actual  distances  can  now  be  obtained 
without  difficulty. 

The  following  table  gives  in  one  column  the  relative  distances 
of  the  planets,  the  Earth's  distance  being  represented  by  1.000. 
In  another  column  it  gives  the  real  distances  in  miles.  They 
are  calculated  according  to  the  most  recent  estimates  of  the 
solar  parallax,  which  will  be  explained  in  the  next  chapter. 

1  These  figures  are  not  exact,  but  will  serve  to  show  the  principles  involved. 


FIG.  15.  —  SURVEYING 
FROM  A  BASE-LINE 


PRINCIPLES   FOR   MEASURING   THE    UNIVERSE    33 


PLANETARY  DISTANCES 

(Solar  Parallax,  8.81") 

RELATIVE 

ACTUAL  (IN  MILES) 

Mercury  . 

.     .          .        .387 

.     .        35,909,000 

.     .          .         .723     . 

.     .        67,087,000 

Earth 

.     .     .            1.000 

.     .     .       1.523 

.     .      141,384,000 

Asteroids 

(    2.080     . 
'1   4.262     . 

.     .      193,000,000 
.     .      395,470,000 

Jupiter    . 

....       5.203     . 

.     .      482,786,000 

Saturn 

....       9.538 

.     .      885,105,000 

Uranus 

....     19.183     . 

.     .  1,779,990,000 

Neptune 

....     30.055     . 

.     .  2,788,800,000 

It  will  be  seen  by  those  who  have  followed  the  argument  thus 
far  that  there  is  no  guessing  about  the  process.  It  is  a  mere 
matter  of  observation  and  calculation.  In  the  first  instance 
given,  the  width  of  the  river  can  be  found  by  stretching  a  cord 
across  it,  or  the  result  can  be  tested  in  various  other  ways.  In 
the  case  of  the  Sun,  Moon,  and  planets,  the  results  can  also  be 
tested  in  other  ways,  as  well  as  by  repeating  the  experiment 
under  different  conditions.  As  soon  as  the  observations  can  be 
carried  out  without  error,  the  distances  can  be  obtained  exactly. 
But  not  before.1 

STAR  DISTANCES 

The  stars  are  too  far  off  for  their  distances  to  be  measured  by 
a  4,000-mile  base-line.  But  as  it  is  found  that  the  Earth  in  Jan- 
uary is  at  an  enormous  distance  from  the  place  which  it  occupies 
in  July,  the  positions  of  the  stars  are  observed  at  both  periods,, 
and  compared  together. 

1  A  few  years  ago  it  was  discovered  that  one  of  the  asteroids,  or  minor  planets, 
which  goes  by  the  name  of  Eros,  moves  in  an  elongated  orbit,  one  part  of  which  is 
nearer  to  us  than  that  of  Mars.  At  certain  periods  this  planet  (which  is  only 
about  twenty  miles  thick)  comes  within  a  distance  of  14,000,000  miles  from  the 
Earth.  By  its  means  celestial  distances  will  before  long  be  much  more  accurately 
known  than  they  are  now, 

8 


34      HOW   TO   KNOW   THE   STARRY   HEAVENS 

This  gives  a  base-line  of  nearly  186,000,000  miles.  But  even 
with  this  gigantic  base  there  are  only  a  few  of  the  nearest  stars 
whose  distances  can  be  even  approximately  estimated.  The 
distance  of  the  nearest  of  them  is  about  135,000  times  as  great 
as  the  length  of  our  enormous  base-line.  It  is  9,000  times  as 
far  off  as  Neptune,  the  outside  planet  in  our  system.  About 
sixty  stars  have  measurable  parallaxes,  a  few  more  have  per- 
ceptible ones,  but  all  the  others  are  at  present  out  of  reach  in 
the  unsoundable  depths  of  infinite  space. 

If  our  eyes  were  as  powerful  and  accurate  as  the  instruments 
of  the  astronomer,  we  could  look  at  a  shining  grain  of  sand 
thirty  miles  away,  and  estimate  its  distance  from  us  by  observ- 
ing how  much  the  eyes  had  to  be  drawn  together  to  focus  on 
the  object. 

The  same  principle  of  triangulation  which  enables  a  surveyor 
to  plot  off  a  township  or  measure  the  height  of  a  mountain  en- 
ables the  astronomer  to  measure  the  world  and  ascertain  the 
distances  of  the  Sun,  Moon,  planets,  and  some  of  the  stars. 
Enormous  as  many  of  the  distances  are,  all  these  measure- 
ments depend  on  an  ordinary  yard-stick,  —  they  are  all  based 
on  the  common  three-foot  rule. 

"E  PUR  SI  MUOVE" 

It  should  be  observed  that  the  above-described  method  of 
measuring  the  distances  of  the  heavenly  bodies  will  in  some 
cases  give  the  same  results  whether  we  suppose  the  Earth  to 
stand  still,  with  the  Sun,  Moon,  planets,  and  stars  swinging 
around  it  once  every  twenty-four  hours,  or  whether  we  suppose 
that  the  diurnal  changes  are  caused  by  the  Earth  revolving  on 
its  axis. 

But,  having  once  found  the  distances,  it  is  evident  that  the 
latter  is  the  true  explanation  of  the  phenomena.  For  if  the 
planet  Neptune  —  distant  as  it  is  —  really  goes  around  the  Earth 
in  a  day,  it  must  go  at  the  unthinkable  speed  of  190,000  miles 
in  a  second  of  time.  And  if  the  stars,  whose  distances  are  so 
much  greater  than  that  of  Neptune,  also  go  around  the  Earth 


PRINCIPLES   FOR   MEASURING   THE   UNIVERSE    35 

every  day,  their  speed  must  be  thousands  and  millions  of  times 
faster  still. 

On  the  other  hand,  if  it  is  the  Earth  that  revolves,  the  motion 
is  nowhere  greater  than  one  mile  in  three  seconds.  The  proba- 
bilities are  evidently  altogether  in  favour  of  the  latter  proposi- 
tion. The  former  one  is  impossible  and  absurd. 

There  is  only  one  way  of  getting  over  the  difficulty.  In  spite 
of  all  who  deny  it,  or  fail  to  realise  it,  the  fact  still  remains  that 
"  the  Earth  does  move." 

MEASURING  THE  PLANETS 

While  measuring  the  distances  of  the  Sun  and  planets,  astron- 
omers have  been  able,  by  measuring  their  apparent  diameters 
(in  degrees,  minutes,  and  seconds  of  arc),  to  ascertain  their  real 
diameters  in  miles.  The  principle  is  a  very  simple  one,  and 
may  be  illustrated  in  this  way. 

A  two-inch  ball  is  8  times  as  large  as  a  one-inch  ball  (2  x 
2x2  =  8).  But  if  a  one-inch  ball  is  viewed  from  a  distance  of 
ten  feet,  it  will  be  just  large  enough  to  hide  a  two-inch  ball 
twice  as  far  away,  or  a  four-inch  ball  four  times  as  far  away. 

Now,  suppose  that  we  have  found  the  Moon  to  be  239,000 
miles  away.  Let  us  get  a  ball  11  feet  5  inches  in  diameter,  and 
place  it  in  a  conspicuous  position  on  the  top  of  a  steep  mountain. 
Having  done  so,  let  us  measure  off  1,262  feet  (which  is  the  one- 
millionth  part  of  the  Moon's  distance),  to  a  place  where  the  ball 
will  come  between  us  and  the  rising  or  setting  Moon.  It  will 
be  found  that  the  ball  is  just  large  enough,  at  that  distance,  to 
hide  the  Moon  from  us.  Now,  as  the  Moon  is  just  a  million 
times  as  far  from  us  as  the  ball  which  hides  it,  it  follows  that 
its  diameter  is  just  a  million  times  greater  (11  feet  5  inches 
X  1,000,000  =  2,162  miles). 

So  far,  so  good.  It  is  interesting  to  note  that,  in  an  eclipse 
of  the  Sun,  the  Moon  acts  the  part  of  the  ball  just  used. 
It  so  happens  that,  while  the  Sun's  average  distance  from  us 
(92,790,000  miles)  is  about  388  times  that  of  the  Moon  (239,000 
miles),  his  diameter  (864,000  miles)  exceeds  hers  (2,162  miles) 


36     HOW  TO   KNOW  THE   STARRY   HEAVENS 

in  about  the  same  proportion.  They  therefore  look  as  though 
they  were  about  the  same  size,  although  the  Sun's  diameter  is 
really  almost  400  times  as  long,  and  his  bulk  is  more  than 
60,000,000  times  as  great  (400  x  400  x  400  =  64,000,000). 

Most  of  the  planets  have  no  measurable  diameter  when  seen 
by  the  naked  eye,  but  by  means  of  the  telescope  their  dimen- 
sions also  have  been  ascertained.  The  stars  cannot  be  measured 
in  this  way,  as  they  are  so  far  off  that  they  have  no  perceptible 
size,  even  when  seen  through  the  most  powerful  telescopes. 
The  amount  of  light  we  receive  from  them  is  almost  the  only 
guide  we  have  to  their  size,  and  even  this  is  of  no  avail  unless 
we  know  something  of  their  distances  from  us. 

WEIGHING  THE  PLANETS 

One  of  the  most  astonishing  things  that  astronomers  have 
been  able  to  do  is  to  weigh  the  Sun  and  planets,  so  as  to  ascer- 
tain their  mass  or  weight.1  Yet  the  principle  is  as  simple  as 
that  used  in  ascertaining  their  dimensions. 

Get  a  light  straight  stick,  and  make  a  'sharp  point  at  each  end 
of  it.  Then  stick  a  potato  on  each  point,  and  hang  the  appa- 
ratus from  the  ceiling  by  a  string.  Shift  the  string  on  the  stick 
till  the  potatoes  balance  one  another.  Now  give  it  a  twirl  and 
release  it.  The  two  potatoes  will  swing  around  the  common 
centre  of  gravity,  where  the  string  is  fastened  to  the  stick. 

If  the  two  potatoes  are  of  the  same  weight,  the  centre  of 
gravity  will  be  the  same  distance  from  each  of  them,  and  it 
will  be  found  that  each  one  swings  around  the  other  one  in  the 
same  sized  circle.  But  if  one  is  heavier  than  the  other,  the 
centre  of  gravity  will  be  nearer  to  the  heavy  one,  and  it  will  be 
found  that  the  small  one  makes  the  largest  circle.  The  appa- 
ratus, indeed,  makes  a  primitive  pair  of  scales  with  which  the 
relative  weight  of  each  potato  can  be  ascertained  by  noting  the 
size  of  the  circle  it  makes. 

1  These  terms  are  not  absolutely  identical.  The  word  mass  refers  to  the  amount 
of  matter  contained  in  anything,  while  weight  has  reference  also  to  the  force  of  gravi- 
tation, which  varies  iu  different  worlds.  The  distinction  is  not  important  here. 


PRINCIPLES   FOR   MEASURING   THE   UNIVERSE    37 

Now  the  Earth  aud  Moon  form  a  similar  weighing-machine. 
They  are  all  the  time  swinging  around  their  common  centre  of 
gravity,  like  our  two  potatoes,  and  their  relative  weights  can  be 
found  by  the  same  process. 

But  at  the  same  time  that  the  Earth  and  Moon  are  swinging 
around  their  common  centre  of  gravity,  the  Earth-Moon  family 
on  the  one  hand,  and  the  Sun  on  the  other,  are  also  swinging 


FIG.  16.  — DAILY  POSITIONS  OF  EARTH  AND  MOON 

It  will  be  seen  that  the  lunar  path  is  always  concave  towards  the  Sun. 

around  their  common  centre  of  gravity.  In  this  case  the  Earth 
and  Moon  together  are  so  small,  in  comparison  with  the  Sun, 
that  they  are  doing  nearly  all  the  swinging.  Nevertheless  the 
Sun  is  doing  his  part  of  the  motion,  even  if  it  is  too  small  to  be 
easily  perceived. 

All  the  members  of  our  Solar  System  (including  even  the 
Sun)  swing  around  their  various  centres  of  gravity  and  influence 
one  another  in  the  same  way,  the  amount  of  their  influence 
depending  on  their  mass  and  distance.  The  Sun  outweighs  all 
the  planets  745  times,  so  that  his  part  of  the  swinging  is  very 
small.  Still  it  exists,  and  although  it  is  convenient  to  say  that 
the  Moon  swings  around  the  Earth,  and  that  the  Earth  swings 
around  the  Sun,  it  would  be  more  correct  to  say  that  the  Earth 
and  Moon  swing  around  their  common  centre  of  gravity,  and 
that  the  Earth-Moon  family  and  Sun  do  the  same. 

It  will  be  seen  that  any  family  of  worlds  can  be  used  as  a 
weighing-machine,  with  which  the  relative  weight  of  each  indi- 
vidual can  be  ascertained  by  its  influence  over  the  other  mem- 
bers of  the  family.  Some  of  the  stars,  even,  can  be  weighed 
against  one  another  when  they  belong  to  one  family. 


38      HOW   TO   KNOW   THE   STARRY   HEAVENS 

ACTUAL  WEIGHT 

When  we  have  found  the  relative  masses  or  weights  of  the 
Sun  and  planets,  we  can,  by  finding  the  actual  weight  of  one  of 
them,  in  tons,  find  the  actual  weight  of  any,  or  all  of  them,  in 
tons. 

The  readiest  way  of  doing  this  is  to  weigh  the  Earth  and  find 
out  how  many  tons  of  material  it  contains.  Of  course  this  is  a 
very  easy  thing  to  do.  All  that  appears  to  be  necessary  is  to 
get  a  very  strong  weighing-machine,  turn  it  upside  down,  so  that 
the  Earth  rests  in  the  pan,  and  then  adjust  the  scale  and  read 
off  the  weight. 

This  last  item  must  not  be  taken  too  seriously.  The  problem 
of  weighing  the  Earth  is  really  one  of  the  most  difficult  tasks 
astronomers  ever  undertook.  It  has  been  solved,  however,  by 
several  different  methods,1  and  it  is  interesting  to  know  that  the 
Earth  weighs  about  6,600  millions  of  millions  of  millions  of 
American  tons  (6,600,000,000,000,000,000,000).  And  the  Sun 
contains  330,000  times  that  amount  of  material. 

1  The  best  plan  is  one  which  employs  a  torsion -balance  to  measure  the  mutual 
attraction  of  lead  balls  at  known  distances  from  one  another.  This  is  then  com- 
pared with  the  observed  attraction  of  the  Earth  for  the  same  lead  balls,  which  are 
known  to  be  at  a  distance  of  about  4,000  miles  from  the  centre  of  attraction.  The 
mutual  attraction  of  the  balls  being  known,  the  law  of  gravitation  shows  how  dense 
the  World  must  be  in  order  to  give  the  balls  their  observed  weight.  (See  Chapter 
XV  for  the  key  to  this  problem. ) 


CHAPTER  IV 

SOME   PROBLEMS   USED  IN  CELESTIAL  MEASUREMENTS 

"The  methods  used  for  measuring  astronomical  distances  are  in  some  applica- 
tions absolutely  the  same  as  the  methods  of  ordinary  theodolite  surveying,  and  are 
in  other  applications  equivalent  to  them.  .  .  .  There  is  nothing  in  their  principles 
which  will  present  the  smallest  difficulty  to  a  person  who  has  attempted  the  common 
operation  of  plotting  from  angular  measures."  —  Sir  George  Airy. 

IN  order  that  the  beginner  may  better  understand  the  prin- 
ciples upon  which  celestial  measures  depend,  a  few  examples 
are  given  here,  going  further  into  details  than  in  the  preceding 
chapter.  I  do  not  undertake  to  say  that  everyone  will  be  able 
to  follow  them  all ;  but  I  have  simplified  them  and  explained 
the  terms  as  much  as  possible,  so  as  to  help  a  non-mathematician 
who  is  willing  to  try. 

DEGREES  IN  A  CIRCLE 

In  trigonometry,  which  deals  with  the  properties  of  three-sided 
figures,  we  (after  the  Greeks)  divide  the  circumference  of  a  circle 
into  360  degrees  of  arc,  denoted  thus  (360°). 

These  degrees  are  indicated  by  straight  lines  radiating  from 
the  centre  of  the  circle,  which  is  supposed  to  be  the  point  of 
observation. 

Where  two  of  these  radiating  lines  enclose  a  square  corner  or 
right-angle,  that  angle  evidently  contains  90°  of  arc  as  measured 
off  on  the  circumference. 

Each  degree  is,  for  convenience,  divided  into  60  minutes  ('), 
and  each  minute  into  60  seconds  (")  of  arc.  These  minutes  and 
seconds  of  arc  have  nothing  whatever  to  do  with  minutes  and 
seconds  of  time.  It  is  a  mere  accident  (and  misfortune)  that 
they  are  called  by  the  same  names. 


40      HOW   TO   KNOW   THE    STARRY    HEAVENS 

Each  line  going  from  the  centre  to  the  circumference  (like  a 
spoke  in  a  wheel)  is  termed  a  radius  (plural  radii). 

RADII  AND  ARC   OF  CIRCLE 

It  has  been  found  that  when  two  radii  are  so  placed  that  the 
central  corner  or  angle  contains  57°  17'  45"  (=  206,265"),  then 
the  arc  of  circle  cut  off  by  the  two  radii  is  just  equal  in  length 
to  one  radius.1  Such  an  arc  is  termed  a  radian. 


FIG.  17.  —  ARC  OF  CIRCLE 

It  naturally  follows  from  this  that  when  the  angle  is  half  of 
that  just  given  then  the  arc  cut  off  is  just  half  as  long  as  each 
radius  (see  Figure  17). 

This  is  expressed  as  follows : 

Let  A  B  represent  arc  of  circle 
"    Z  "        size  of  angle 

"    Y  "        radius  of  circle 

"   X  «        angle  of  57°  17'  45" 

Then  when  Z  =  X,  A  B  =  Y 

When  Z  =  JX,  A  B  =  j Y 

WhenZ  =  JX,  A  B  =  JY 

And  so  on. 

1  That  is,  the  part  of  the  tire  which  is  cut  off  would  be,  if  straightened  out, 
just  as  long  as  one  of  the  spokes, 


PROBLEMS   IN   CELESTIAL  MEASUREMENTS    41 

The  result  is,  that  if  we  know  the  distance  of  an  object  in  miles, 
we  can  tell  its  diameter  in  miles  by  measuring  the  angle  enclosed 
by  its  opposite  sides.  For  example,  if  an  object  15  miles  away 
is  long  enough  to  subtend  an  angle  of  3°  49'  11"  (=ya^  of  X), 
then  it  must  be  about  a  mile  long. 

On  the  other  hand,  if  we  know  the  diameter  of  an  object  in 
miles,  we  can  tell  its  distance  from  us  in  miles.  For  example, 
if  we  know  that  a  certain  object  is  a  mile  long,  and  we  find  by 


FIG.  18.  —  CHORD  OF  ARC 

our  instruments  that  it  subtends  an  angle  of  3°  49'  11"  (=  &  of  X), 
then  it  must  be  about  15  miles  away. 

These  two  things  can  be  ascertained;  however,  only  when  the 
distant  object  is  near  enough  to  have  a  measurable  size  when 
seen  through  a  telescope. 

THE  CHORD  OF  AN  ARC 

For  many  purposes  it  is  convenient  to  draw  a  straight  line 
cpnnecting  the  outer  ends  of  the  two  radii.  This  line  is  called 
a  chord,  and  the  three  straight  lines  together  form  what  is  known 
as  a  triangle  (see  Figure  18). 

In  such  a  triangle  the  two  outside  corners  or  angles,  A  and  B, 
are  equal  to  one  another,  and  are  each  sharper  or  more  acute 
than  a  right  angle.  This  is  true  whether  the  centre  angle  Z  be 


42     HOW   TO   KNOW  THE   STARRY   HEAVENS 

great  or  small.  In  fact  the  greater  the  central  angle  is,  the  more 
acute  become  the  outer  angles.  It  is  useful  to  remember  that  if 
the  number  of  degrees  in  all  the  three  angles  of  a  triangle  be 
added  together,  they  are  always  equal  to  the  number  contained 


FIG.  19.  —  SINE  OF  ANGLE 


in  two  right  angles.  That  is,  they  always  contain  exactly  180° 
of  arc. 

The  chord  of  an  arc  is  of  course  shorter  than  the  arc  with 
which  it  begins  and  ends.  The  smaller  the  angle,  the  less  differ- 
ence there  is  between  the  two,  and  in  very  small  angles  this 
difference  can  be  neglected,  it  is  so  minute. 

The  Greeks  made  great  use  of  chords  in  their  investigations. 
Ptolemy,  the  astronomer  (/.  127-151  A.  D.),  constructed  tables 


PROBLEMS   IN   CELESTIAL  MEASUREMENTS    43 


SHORE. 


giving  the  length  of  both  arcs  and  chords  for  every  half-degree 
up  to  two  right  angles. 

THE  SINE   OF  AN  ANGLE 

The  Hindus,  however,  simplified  their  problems  by  taking 
the  chord  of  double  the  angle,  and  then  cutting  it  in  two  and 
discarding  one  half.  The 
half-chord  used  (A  B)  is 
known  as  a  sine  of  the  angle 
(A  Z  B)  it  measures  (see 
Figure  19). 

The  advantage  of  a  sine 
over  a  chord  is  this:  In 
solving  problems  in  trigo- 
nometry it  is  often  conven- 
ient to  have  one  of  the  outer 
angles  a  right  angle,  that  is, 
one  containing  90°  of  arc. 
Now,  as  the  chord  (A  F)  is 
cut  in  the  middle  by  the 
bisecting  radius  (Z  B),  the 
two  lines  always  cross  at 
right  angles  (at  B),  and  the 
resulting  triangle  (A  B  Z)  is 
a  right-angled  one.  The 
result  of  one  of  the  angles 
being  invariable  is  that  part 
of  the  labour  is  saved,  as  the 
calculations  are  confined  to 
the  other  two  angles.  All 

books  on  trigonometry  have  tables  giving  the  length  of  sines, 
etc.,  for  every  degree  and  fraction  of  a  degree. 

MEASURING  INACCESSIBLE  DISTANCES 

The  advantage  of  a  right-angled  triangle  is  shown  in  the  fol- 
lowing problem,  which  is  something  similar  to  one  given  in  the 
preceding  chapter : 


BASE 


YDS'. 


FIG.  20.  —  MEASURING  WIDTH  OF 
RIVER 


44      HOW   TO   KNOW   THE   STARRY   HEAVENS 

A  surveyor  wishes  to  measure  the  width  of  a  river  without 
crossing  to  the  other  side  (see  Figure  20). 

First  he  measures  off,  on  his  own  side  of  the  river,  a  base-line 
(A  B)  10  yards  long.  He  stands  at  one  end  (B)  of  his  base- 
line, and  points  his  instrument  at  the  other  end  of  it  (A).  He 
then  turns  the  instrument  one  quarter  round  (90°  of  arc),  and 
selects  an  object  (Z)  on  the  opposite  bank  of  the  river.  Having 
made  a  note  of  the  number  of  degrees  he  has  turned  the  instru- 
ment (90°),  he  goes  over  to  the  other  end  (A)  of  the  base-line 
and  repeats  the  operations.  He  will  find  that  he  will  not  have 
to  turn  his  instrument  so  far  around  to  make  it  point  to  the 
object  selected  (Z). 

Let  us  suppose  that  he  has  to  turn  it  only  89°.  The  two 
angles  at  the  base  will  then  together  equal  179°.  Now,  as  the 
three  angles  of  a  triangle,  added  together,  always  equal  two 
right  angles  (180°),  it  is  obvious  that  the  opposite  angle  (Z) 
must  be  just  1°. 

The  problem  then  stands  as  follows : 

As  the  sine  of  1°  (angle  Z) 
Is  to  the  sine  of  89°  (angle  A), 
So  is  10  yards  (length  of  base  A  B) 
To  the  perpendicular  Z  B  (or  Y). 

After  obtaining  the  length  of  the  sines  of  1°  and  89°  (which 
are  given  in  all  books  on  trigonometry),  the  problem  is  solved 
as  follows : 

As  01745  is  to  99985,  so  is  10  yards  to  the  answer,  573  yards,  — 

which  is  the  width  of  the  river  as  exactly  as  it  can  be  found 
with  five-place  logarithms. 

PARALLAX 

The  angle  (Z)  opposite  the  base  of  such  a  triangle  is  called 
by  surveyors  and  astronomers  the  parallax  of  the  distant  object. 
The  further  off  the  object  is,  the  smaller  becomes  its  parallax. 


PROBLEMS   IN   CELESTIAL   MEASUREMENTS     45 

The  longer  the  base  from  which  it  is  measured,  the  larger 
becomes  the  parallax.1 

In  the  problem  just  considered  it  is  obvious  that  if  the  object 
Z  is  exactly  west  of  the  observer  at  B,  it  will  no  longer  be  ex- 
actly west  of  him  when  he  goes  to  A.  It  will  be  a  little  to  the 
south  of  west.  It  is  also  obvious  that  the  amount  of  its  displace- 
ment will  depend  on  its  distance  from  him.  The  farther  off  the 
object  is,  the  less  it  is  displaced  when  he  goes  from  one  end  of 
the  base  to  the  other.  In  other  words,  as  stated  before,  the 
more  distant  the  object  is,  the  smaller  becomes  its  parallax.  If  it 
is  a  very  long  way  off,  it  may  appear  to  be  exactly  west  from  both 
ends  of  the  base-line.  In  this  case  it  will  be  necessary  to  greatly 
lengthen  the  base-line  in  order  to  measure  the  distance  of  the 
object. 

The  word  "  parallax  "  is  rather  a  hard  one  to  remember.  But 
astronomers  can  simplify  matters  when  referring  to  the  Sun's 
parallax  by  calling  it  the  "  mean  equatorial  long  horizontal  solar 
parallax."  This  is  a  useful  thing  to  know. 

A  few  examples  of  how  astronomers  utilize  these  principles 
will  conclude  this  chapter,  which  some  may  consider  to  be  dryer 
than  a  California  summer,  and  more  uninteresting  than  a 
Baedeker's  guide-book  to  one  who  never  travels. 

MEASURING  THE  MOON'S  DISTANCE 

We  will  of  course  start  with  the  problem  of  finding  the  Moon's 
distance  from  the  Earth. 

In  Figure  21  the  large  circle  represents  a  section  of  the  Earth 
through  the  two  poles  of  rotation.  The  small  circle  in  the  dis- 
tance represents  the  Moon.  B  and  L  are  the  two  stations  at  the 
ends  of  the  measured  base-line  B  L.  We  will  imagine  that  they 
are  4,000  miles  apart,  and  that  they  are  both  on  the  same  meri- 
dian and  at  the  same  distance  from  the  equator,  which  lies 
between  them. 

1  "  Parallax  may  be  defined,  generally,  as  the  change  produced  in  the  apparent 
place  of  an  object  when  it  is  viewed  from  a  point  other  than  that  of  reference."  - 
ENCY.  BRIT.,  Parallax. 


46      HOW  TO  KNOW   THE   STARRY   HEAVENS 

Let  us  suppose  that,  to  the  observer  at  L,  the  Moon's  centre  is 
exactly  on  the  celestial  equator,  and  is  therefore  exactly  90°  from 
the  south  pole  (as  well  as  from  the  north  pole). 

Then,  to  the  observer  at  B,  the  Moon's  centre  will  be  found  to 
be  57  minutes  of  arc  (nearly  1°)  south  of  the  equator.  That  is 
to  say,  it  will  be  only  89°  3'  from  the  south  pole,  of  which  the 
position  is  known  even  when  it  is  out  of  sight.  In  the  triangle 
B  L  M,  therefore,  the  angles  B  and  L  lack  57'  of  making  two 


FIG.  21. —MEASURING  DISTANCE  OF  MOON 

right  angles  (180°).     This  57'  is  evidently  the  size  of  the  other 
angle  M. 

We  have  now  a  problem  absolutely  identical  in  principle  with 
the  preceding  one,  which  dealt  with  the  width  of  a  river.  It 
stands  as  follows: 

As  the  sine  of  51'  (the  angle  M) 

Is  to  the  sine  of  89°  3'  (the  angle  B), 

So  is  4,000  miles  (the  length  of  base  B  L) 

To  the  perpendicular  L  M. 

The  problem  is  easily  solved  with  almost  the  same  figures  as 
in  that  dealing  with  the  width  of  the  river. 

As  01658  is  to  99986,  so  is  4,000  miles  to  240,000  miles,  — 

which  is  the  approximate  distance  of  the  Moon  at  the  time  when 
the  angles  were  measured.1 

1  The  above  problem  has  been  simplified  as  much  as  possible,  so  that  the  prin- 
ciple may  be  readily  grasped.  As  a  matter  of  fact,  the  two  ends  of  the  base-line 


PROBLEMS  IN   CELESTIAL  MEASUREMENTS    47 

For  convenience  of  comparison,  all  results  are  reduced  to  fit  a 
right-angled  triangle  having  a  base-line  equal  to  the  radius,  or 
semi-diameter,  of  the  Earth  at  the  equator.  The  parallax  is 
then  known  as  the  horizontal  equatorial  parallax. 

The  distances  of  Mars  and  Eros  are  measured  in  the  same 
way  as  that  of  the  Moon.  But  in  the  case  of  the  Sun  the 
results  have  to  be  obtained  by  taking  advantage  of  a  transit  of 


Y 


FIG.  22.  —  ARC  OF  CIRCLE 

Venus,  or  by  some  other  indirect  method.  The  principles  and 
results  are,  however,  the  same. 

ARC  PROBLEMS 

The  five  following  problems  are  worked  out  by  means  of  two 
radii  and  the  enclosed  arc  (see  Figure  22).  The  observer  is 
supposed  to  be  at  Z. 

PROBLEM  I.  —  Given  the  angular  diameter  of  Sun  or  Moon, 
32'  of  arc  (=1920"  of  arc),  as  seen  from  the  Earth,  find  their 
distances  from  us  in  terms  of  their  own  diameter  A  B  (=  1). 

are  never  exactly  on  the  same  meridian.  Nor  are  they  ever  exactly  the  same  dis- 
tance north  and  south  of  the  equator.  And  the  distance  required  is  not  from  the 
Moon  to  the  station  L,  but  from  the  centre  of  the  Moon  to  the  centre  of  the  Earth. 
Quite  a  number  of  corrections  and  precautions  have  to  be  taken  to  give  trust- 
worthy results.  But  they  need  not  be  given  here.  It  is  sufficient  if  the  principle 
of  the  problem  is  thoroughly  understood. 


48     HOW  TO   KNOW  THE   STARRY  HEAVENS 

X  20fi  265" 

2  A  B  =  Y.    Therefore    l  J2(r    A  B  =  107  A  B. 

[NOTE.  —  For  meaning  of  letters  see  the  table  at  page  40.] 

So  that  the  distances  of  the  Sun  and  Moon  from  the  Earth  are 
both  alike  in  terms  of  their  diameters ;  that  is  to  say,  the  dis- 
tance of  each  one  of  them  is  107  times  as  great  as  its  actual 
diameter,  whatever  that  may  be. 

This  problem  does  not  tell  us  how  far  they  are  from  us  or  how 
large  they  are.  It  merely  proves  that  if  the  Sun  (or  Moon)  is  a 
mile  across,  then  it  is  107  miles  away  from  us ;  while  if  it  is 
1,000,000  miles  across,  then  it  is  107,000,000  miles  away. 

PROBLEM  II.  —  Given  the  real  distance  of  the  Sun,  92,790,000 
miles,  and  its  angular  diameter,  32'  of  arc  (=1,920"),  find  its 
real  diameter  in  miles. 

MILE$  MILKS 

Z  1    920 

^ Y  =  A  B.    Therefore  ^265  92>790>000  =  863,727 

The  following  form  of  this  problem  may  perhaps  be  better 
understood : 

As  X  is  to  Z,  so  is  Y  to  A  B. 

Worked  out,  this  is  as  follows : 

MILKS  MILES 

As  206,265" :  1,920"  : :  92,790,000  : 863,727 

PROBLEM  III.  —  Given  the  real  distance  of  the  Moon,  239,000 
miles,  and  its  angular  diameter,  31'  6"  (=  1,866"),  find  its  real 
diameter  in  miles. 

This  is  a  similar  problem  to  the  preceding  one. 

MILES  MILES 

rj  1    Q  A  A 

=£  Y  =  A  B.    Therefore  2^265  239>000  =  2>162 
Or,  by  the  "  rule  of  three  " : 

MILES  MILES 

As  206,265"  :  1,866"  : :  239,000  : 2,162 


PROBLEMS   IN   CELESTIAL  MEASUREMENTS    49 

PROBLEM  IV.  —  Given  the  real  diameter  of  the  Sun,  863,727 
miles,  and  its  angular  diameter  32'  (— 1,920"),  find  its  real  dis- 
tance in  miles. 

It  will  be  seen  that  this  is  the  reverse  of  Problem  II. 

MILES  MILES 

X  206,265 

~  A  B  =  Y.    Therefore  863,727  =  92,790,000 


Or,  as  before : 

MILES  MILKS 

As  1,920"  :  206,265"  : :  863,727  :  92,790,000 

PROBLEM  V.  —  Given  the  real  diameter  of  the  Moon,  2,162 
miles,  and  its  angular  diameter,  31'  6"  (=  1,866"),  find  its  real 
distance  in  miles. 

It  will  be  seen  that  this  is  the  reverse  of  Problem  III.  It  is 
worked  the  same  as  Problem  IV. 

MILES  MILES 

^A  B  =  Y.     Therefore  ^f|fjf  2,162  =  239,000 

SINE  PROBLEMS 

The  following  problems  are  worked  out  by  means  of  a  right- 
angled  triangle  constructed  of  two  radii  and  the  sine  (or  half- 
chord)  of  the  enclosed  angle  (see  Figure  23  ;  lettering  same  as 
before). 

PROBLEM  VI. —  Given  the  Sun's  distance,  92,790,000  miles, 
and  angular  semi-diameter  16'  (=  960"),  find  its  real  semi- 
diameter  in  miles. 

This  is  like  Problem  II,  except  that  the  semi-diameter  is  used 
instead  of  the  diameter. 

MILKS  MILES 

Z  960" 

Y  Y  =  A  B.     Therefore  9nr  9r-»  92,790,000  =  431,863 

.A.  ^UDj^OiJ 

PROBLEM  VII.  —  Given  the  Moon's  distance,  239,000  miles, 
and  its  angular  semi-diameter,  15'  33"  (=  933"),  find  its  real 
semi-diameter  in  miles. 

4 


50     HOW   TO   KNOW   THE   STARRY   HEAVENS 

This  is  like  Problem  III,  but  uses  the  semi-diameter  instead 
of  the  diameter. 

MILKS  MILES 

Z  933" 

Y  =  A  B.    Therefore  //  239,000  =  1,081 


FIG.  23. —  SINE  OF  ANGLE 


In  all  the  following  problems  the  observers  are  supposed  to  be 
at  A  and  B.  Z  is  supposed  to  be  the  centre  of  the  celestial  body 
under  observation. 

PROBLEM  VIII.  —  Given  the  Sun's  parallax,  8.81",  and  the 
Earth's  semi-diameter,  3,963  miles,  find  the  Sun's  distance  in 
miles. 


PROBLEMS   IN   CELESTIAL   MEASUREMENTS     51 

All  solar  and  planetary  parallaxes  are  for  convenience  reduced 
to  fit  the  semi-diameter  of  the  Earth.  This  is  here  represented 
by  A  B,  and  the  parallax  by  the  opposite  angle  Z. 

MILES  MILKS 

X  206  265" 

£-  A  B  =  Y.     Therefore     8'8]//     3,963  =  92,790,000 

PROBLEM  IX.  —  Given  the  Moon's  parallax,  57'  (=  3,420'% 
and  the  Earth's  semi-diameter,  3,963  miles,  find  the  Moon's  dis- 
tance in  miles. 

This  is  a  similar  problem  to  the  preceding  one. 

MILES  MILES 

X  206,265" 

2~  A  B  =  Y.     Therefore   3  ^2Q,    3,963  =  239,000 

Here  is  the  same  problem  worked  by  logarithms,  which  must 
be  obtained  from  published  tables  of  logarithms  : 

As  the  sine  of  57'  (angle  Z)  8.21958 
Is  to  base  3,963  miles  (A  B)  3.59802 
So  is  sine  of  89°  3'  (angle  A)  9.99994 

13.59796 
8.21958 

To  perpendicular  (Z  B)  5.37838  =  239,000  miles 

[NOTE.  —  In  small  angles  Z  B  =  Y.] 

PROBLEM  X.  —  Given  the  Sun's  distance,  92,790,000  miles,  and 
the  Earth's  semi-diameter,  3,963  miles,  find  the  Sun's  parallax. 
This  is  the  reverse  of  Problem  VIII. 

4j?  X  =  Z.     Therefore  ^fg^O  206>265"  =  8'81° 

PROBLEM  XI.  —  Given  the  Moon's  distance,  239,000  miles,  and 

the  Earth's  semi-diameter,  3,963  miles,  find  the  Moon's  parallax. 

This  is  the  reverse  of  Problem  IX,  and  similar  to  Problem  X, 


52     HOW  TO   KNOW   THE   STARRY   HEAVENS 

A  B  3,963 

~~  X  =  Z.    Therefore  206>265"  =  57' 


[If  it  is  not  too  late,  I  would  here  suggest  that  those  who  do 
not  like  Mathematics  would  do  well  to  "  skip  "  the  foregoing 
chapter.] 


FIG.  24.  —  SUN,    SHOWING  SPOTS  AND  FACUL^C 
Photographed  at  Greenwich  Observatory,  Feb.  13,  1892. 


V 


CHAPTER  V 

THE  CHARIOT  OF  IMAGINATION 

"  Before  their  eyes  in  sudden  view  appear 
The  secrets  of  the  hoary  deep  ;  a  dark 
Illimitable  ocean,  without  bound, 

Without  dimension,  where  length,  breadth,  and  height, 
And  time,  and  place,  are  lost."  —  Milton,  "  Paradise  Lost,"  Book  IL 

"  Oh  Deep,  whose  very  silence  stuns  ! 
Where  Light  is  powerless  to  illume, 
Lost  in  immensities  of  gloom, 
That  dwarf  to  motes  the  flaring  suns  ! " 

—  G.  Sterling,  "  The  Testimony  of  the  Suns." 

A  LONG  JOURNEY 

TO  enable  us  to  realise,  to  some  extent,  what  position  man 
holds  with  reference  to  the  Universe,  let  us  leave  our 
Earth  for  a  short  time,  and  hasten  away,  in  the  Chariot  of  Im- 
agination, to  a  point  in  space  half-way  between  our  Sun  and 
Alpha  Centauri,  the  nearest  of  the  other  stars. 

We  will  take  with  us  a  specially  constructed  chronometer, 
made  to  indicate  long  periods  of  time ;  a  special  cyclometer,  made 
to  fit  the  wheels  of  our  chariot ;  and  a  number  of  other  scientific 
instruments  which  may  prove  useful  in  our  celestial  researches. 

In  order  that  we  may  not  get  lost  or  have  any  corners  to  turn, 
we  make  our  start  from  Cape  Town,  South  Africa,  in  the  night- 
time, when  the  Moon  is  above  the  horizon  and  Alpha  Centauri 
is  on  the  meridian. 

As  this  may  be  the  first  time  some  of  us  have  travelled  through 
space  unaccompanied  by  Mother  Earth,  it  will  be  well  for  us  to 
travel  slowly,  so  that  we  can  get  a  good  view  of  our  surroundings, 
and  at  the  same  time  avoid  running  into  unnecessary  danger. 
We  will  therefore  keep  a  firm  hand  on  the  lines,  from  the  start, 


54     HOW  TO   KNOW  THE   STARRY   HEAVENS 

so  as  to  prevent  our  imaginary  steeds  from  running  away  with 
us.  On  such  a  long  journey  the  most  satisfactory  speed  for  us 
to  keep  up  will  perhaps  be  that  at  which  light  travels,  about 
186,000  miles  per  second. 

Everything  being  ready  for  our  trip,  the  signal  is  given.  "  One, 
two,  three.  Away  we  go  I " 

Before  the  words  are  fairly  uttered,  we  find  ourselves  at  the 
Moon's  distance  and  in  the  bright  sunshine.  By  a  curious 
optical  delusion  we  did  not  seem  to  move  when  the  word  was 
given,  but  the  moonlit  Earth  suddenly  dropped  from  beneath 
our  feet.  For  a  fraction  of  a  second  it  appeared  to  swell,  as  dis- 
tant lands  and  moonlit  seas  sprang  above  the  horizon.  Then 
the  Sun  rose  with  a  jerk,  and  the  crescent  Earth  began  to  shrink 
in  size  as  its  distance  increased.1 

At  the  end  of  one  minute  the  Earth  is  still  plainly  visible  in 
the  bright  sunlight,  but  the  Moon  is  almost  out  of  sight.  In 
five  minutes  nothing  is  to  be  seen  of  either  of  them. 

For  a  time  we  are  in  the  sunshine,  but  the  light  soon  begins 
to  wane  as  we  recede  from  the  Sun  and  approach  the  confines  of 
the  Solar  System. 

In  four  hours  we  are  at  the  distance  of  Neptune,  and  the  Sun 
is  not  much  more  than  a  very  brilliant  star  in  the  gathering 
twilight. 

Although  the  Sun  continues  to  dwindle  in  size  as  we  leave  it 
behind,  it  shines  continuously,  there  being  no  horizon  to  hide  it 
from  us.  After  we  have  been  a  month  on  our  journey,  as  shown 
by  our  chronometer,  it  is  practically  nothing  but  a  bright  star 
among  the  multitude  of  stars  by  which  we  are  entirely  sur- 
rounded. 

By  this  time  we  have  discovered  that  we  have  left  behind 
many  things  which  seemed  very  real  and  important  while  we 
remained  on  terra  firma. 

There  is  now  no  north  or  south,  no  up  or  down.     The  star- 

1  For  the  above  effects  to  be  produced,  we  should  really  have  to  travel  slower 
than  light,  otherwise  nothing  would  be  visible  in  our  rear.  Every  impossible 
illustration  has  its  discrepancies,  as  Artemus  Ward  would  say, 


THE   CHARIOT   OF   IMAGINATION  55 

sphere  has  ceased  to  turn  on  its  axis,  so  that  there  are  no  pole 
stars.  The  wandering  planets  have  long-  since  disappeared. 
There  is  no  Sun  or  Moon.  Day  and  night  have  ceased  to  roll. 
Seed-time  and  harvest  come  no  more.  Summer  and  winter  are 
meaningless  terms.  Aside  from  our  chronometer,  months,  years, 
and  centuries  have  now  no  significance.  Away  from  our  Earth, 
geological  periods  trouble  the  mind  no  more. 

We  keep  on  in  a  straight  line,  at  the  same  speed,  for  two  long 
years>  as  registered  by  our  chronometer.  The  star  which  used 
to  be  our  Sun  is  now  directly  behind  us,  but  has  long  ceased  to 
be  conspicuous  for  either  size  or  brilliancy. 

And  now  the  special  cyclometer  which  we  brought  along  tells 
us  that  we  are  at  last  nearing  our  goal,  the  half-way  house 
between  the  centre  of  our  system  and  the  nearest  outside  star. 

AMONG  THE  STARS 

Arrived  at  our  lonely  destination,  let  us  check  our  imaginary 
horses,  hitch  them  to  an  imaginary  post,  and  take  a  survey  of 
our  actual  surroundings. 

As  we  did  not  bring  our  Earth  along  with  us,  our  view  is  not 
impeded  in  a  downward  direction.  We  can  see  clearly  below 
and  around,  as  well  as  above. 

What  is  there  to  be  seen  from  our  point  of  vantage  ? 

One  of  the  first  things  to  attract  our  attention  is  that  we  do 
not  appear  to  have  any  immediate  surroundings.  We  are  soli- 
tary in  empty  space. 

Instead  of  being  surrounded  by  houses  and  trees  lighted  up 
by  the  dazzling  glare  of  a  hot  Sun,  or  half-revealed  by  the  soft 
glamour  of  a  pale  Moon,  we  find  ourselves  alone  in  the  midst  of 
perpetual  starlight,  which  no  Sun  or  Moon  ever  interferes  with. 

There  is  no  cloud  or  fog,  for  all  is  cold,  clear,  still,  dark,  and 
apparently  void. 

But  in  the  far  distance  there  are  plenty  of  objects  to  make  up 
for  an  unoccupied  "  fore-space." 

The  "  back-space "  of  sky  is  all  more  or  less  crowded  with 
stars.  To  the  naked  eye  there  are  about  6,000  visible.  These 


56     HOW   TO   KNOW  THE  STARRY   HEAVENS 

are  distributed  promiscuously  in  irregular  clusters  and  hap- 
hazard groups,  without  any  regard  to  pattern  or  symmetry. 

But  besides  these  groups  of  stars  there  are,  in  some  parts  of 
the  sky,  great  irregular  streaks  of  nebulous  haze.  One  set  of 
these  hazy  streaks  is  so  long-drawn-out  that  its  snake-like  folds 
and  spirals  almost  form  a  girdle  around  us. 

The  unaided  eye  cannot  pierce  this  haze,  and  without  further 
insight  even  the  imagination  itself  is  unable  to  invent  a  reason- 
able explanation  of  this  "  Milky  Way." 

THE  EYES  OF  SCIENCE 

Let  us  now  imagine  that  our  eyes  improve  in  light-grasping 
power  till  they  equal  the  most  powerful  telescopes  in  existence. 
What  is  there  now  to  be  seen  from  our  point  of  vantage  ? 

The  result  is  something  startling  —  astounding  —  overwhelm- 
ing. The  scene  is  grand  beyond  the  power  of  language  to 
describe  —  magnificent  beyond  the  ability  of  the  mind  to 
conceive. 

The  Earth  we  came  from  is  still  invisible  —  lost  in  the  depths 
of  infinite  space. 

The  Sun  that  ruled  our  Solar  System  with  such  undisputed 
sway  is  visible  still,  but  it  rules  no  more.  It  was  a  SUN  that 
reigned  supreme  among  a  thousand  little  twinkling  stars.  It  is 
now  but  a  star  among  a  hundred  million  fellow-stars. 

But  though  we  have  lost  our  Earth  and  its  Sun,  we  have 
gained  more  than  we  have  lost.  For  we  have  revealed  before 
us  a  goodly  portion  of  the  Universe  itself.  And  though  we 
see  no  more  a  panoramic  succession  of  days  and  nights,  seasons 
and  years,  we  do  not  miss  these  earthly  phenomena.  For  in  their 
stead  we  see  the  stately  evolutions  of  countless  squadrons  of 
heavenly  orbs,  circling  through  never-ending  time  in  an  ocean  of 
limitless  space. 

We  have  here  no  need  of  the  Sun,  neither  of  the  Moon ;  for 
the  everlasting  glory  of  the  Great  Cosmos  enlightens  us,  and  the 
iridescent  mantle  of  Universal  Nature  enfolds  us. 


FIG.   26.  -   SOLAH  FLAMES  AND  COHONA,   AS  SEEN   DUKIXG   ECLIPSK  OF 
MAY  28,   1900 

By  Burckhardt.     (From  Comstock's  "  Text-book  of  Astronomy,'11  published  by 
Messrs.  D.  Applelon  &  Co.) 


VTBRAT7 

or  THE 

UNIVERSITY 

or 

JzALirCi* 


THE   CHARIOT  OF  IMAGINATION  57 

On  every  side,  above  and  below,  we  see  stars  by  the  million. 
They  are  strewn  through  endless  space  like  the  blinding  snow- 
flakes  of  a  Western  blizzard.  They  are  as  thick  as  the  leaves 
of  an  earthly  forest. 

And  we  know  that  each  and  every  star  is  a  SUN,  more  or  less 
like  unto  our  Sun.  Many,  if  not  all  of  them,  have  subject 
worlds  revolving  around  them  like  the  planets  which  compose 
our  own  system. 

Gazing  on  such  a  picture,  words  are  not  equal  to  express  our 
sense  of  littleness.  As  the  poet  says : 

This  is  a  wondrous  sight, 

And  mocks  all  human  grandeur." 

Contemplating  the  star-strewn  heavens,  the  deist  may  well 
exclaim  with  one  of  old  — 

"  When  I  contemplate  the  heavens, 

The  work  of  thy  hands, 
The  Moon  and  the  stars, 

That  thou  hast  disposed, 
What  is  Man, 

That  thou  shouldst  remember  him, 
The  Son  of  Man, 
That' thou  shouldst  watch  over  him  ? " 

—  Ps.  viii,  3  (Segond  and  Diodati). 

The  poet  Shelley  has  beautifully  described  such  a  scene.     He 

says: 

"  Below  lay  stretched  the  Universe. 
There,  far  as  the  remotest  line 
That  bounds  imagination's  flight 
Countless  and  unending  orbs 
In  mazy  motion  intermingled, 
Yet  each  fulfilled  immutably 
Eternal  Nature's  law. 
Above,  below,  around, 
The  circling  systems  formed 
A  wilderness  of  harmony ; 
Each  with  undeviating  aim, 
In  eloquent  silence,  through  the  depths  of  space, 
Pursued  its  wondrous  way." 


58     HOW  TO  KNOW  THE  STARRY  HEAVENS 

All  around  us  is  — 

—  "  the  abyss  of  an  immense  concave, 
Radiant  with  million  constellations,  tinged 
With  shades  of  infinite  colour." 

A  RUSH  THROUGH  SPACE 

After  having  gazed  for  a  while  at  the  wonderful  scene  around 
us?  —  a  scene  so  magnificent  that  even  the  words  of  a  Saul  among 
the  poets  fail  to  give  any  adequate  conception  of  it,  —  we  un- 
hitch our  imaginary  horses  from  our  imaginary  post,  turn  our 
Chariot  of  Imagination  toward  one  of  the  stars,  and  rapidly 
approach  it. 

ONLY  A  STAR 

The  star  we  selected  for  examination  was  a  very  ordinary- 
looking  star.  It  was  far  smaller  than  many  of  its  neigh- 
bours, and  did  [not  shine  anything  like  so  brightly  as  some  of 
them. 

But  now  that  we  have  arrived  in  its  vicinity  it  has  grown  in 
size  and  brilliancy  till  all  the  other  stars  have  either  gone  out 
of  sight  or  become  faint  dots  of  light,  just  perceptible  in  the 
growing  daylight.  It  has,  indeed,  become  so  overwhelmingly 
radiant  that  we  have  to  put  on  dark  spectacles  to  enable  us  to 
use  our  eyes  without  being  blinded. 

Let  us  stop  and  watch  this  star  for  a  thousand  years  or  so,  and 
see  what  changes  are  going  on  around  it  as  it  drifts  along  in  the 
ocean  of  space. 

The  star  itself  is  a  round  yellowish-white  ball  more  than 
800,000  miles  in  diameter.  Its  glowing  surface,  or  photosphere, 
is  one  vast  mass  of  shining  cloud,  which  has  the  appearance  of 
being  dotted  all  over  with  still  brighter  specks,  like  rice-grains. 
This  cloud-like  photosphere  is  composed  of  calcium  and  other 
elements,  kept  in  a  white-hot  state  by  an  unimaginably  intense 
heat  rising  from  the  gaseous  interior  of  the  star.  The  white 
photosphere  radiates  into  outer  space  about  four  times  as  much 


FIG.  25.  —  GROUP  OF  SUNSPOTS 

Photographed  with  the  Greenwich  26-inch  Refractor,  on  Sept.  11,  1898.     The  largest 
nucleus  was  about  24,000  miles  long. 


FIG.  27.  —  ERUPTIVE  PROMINENCES 

Eclipse  of  May  28,  1900  (Barnard  and  Ritchey).     The  largest  of  these  "  hydrogen  flames  ' 
is  60,000  miles  high. 


THE   CHARIOT  OF  IMAGINATION  59 

light  and  heat  as  an  electric  arc-light  of  the  same  size  would 
do.1 

There  are  some  peculiar  features  about  this  star  as  seen  from 
a  short  distance.  Physical  and  mechanical  reactions  of  incon- 
ceivable violence  are  taking  place  beneath  its  surface.  In  some 
places  they  give  rise  to  what  look  like  volcanic  eruptions  on  a 
vast  and  awe-inspiring  scale.  To  an  outside  observer  these 
centres  of  eruption  appear  like  great  irregular  black  blotches 
scattered  about  the  white  cloud-like  photosphere.  They  are  sur- 
rounded by  plume-like  shadows  or  penumbrae.  By  watching 
these  black  spots  we  soon  find  that  the  star  is  spinning  slowly 
around  on  its  axis,  completing  a  revolution  in  about  twenty-seven 
of  our  days. 

The  light  given  out  by  this  white  photosphere  is  so  dazzling 
that  little  more  can  be  made  out,  even  with  dark  glasses.  We  will 
therefore  use  special  instruments  to  turn  it  aside,  so  as  to  enable 
us  to  see  more  clearly  what  other  phenomena  are  going  on  in 
the  neighbourhood. 

We  can  now  see  that  the  entire  body  of  the  star  is  buried 
under  a  shoreless  ocean  of  transparent  fire  of  a  scarlet  hue.  This 
fiery  ocean,  or  atmosphere,  is  everywhere  from  4,000  to  5,000 
miles  deep,  and  appears  to  rest  on  the  white  cloud-like  photo- 
sphere already  described.  The  storm-tossed  surface  of  this  fiery 
ocean  bristles  at  every  point  with  huge  ascending  "  flames  "  of  the 
same  scarlet  colour.  Those  on  our  side  of  the  star  are  not  readily 
examined,  on  account  of  the  brilliancy  of  the  photosphere,  but 
those  around  the  edges  are  plainly  visible  with  proper  apparatus. 
Most  of  them  are  about  8,000  or  10,000  miles  high,  but  here  and 
there  are  larger  ones,  reaching  up  60,000  miles  or  more.  These 
huge  ruddy  flames  assume  a  great  variety  of  forms.  They  re- 
semble jets  of  steam,  fireworks,  fountains,  ocean  breakers,  cy- 
clones, torpedo  explosions,  and  volcanic  eruptions,  all  on  a  scale 
of  inconceivable  magnitude.' 

1  By  the  way,  an  electric  arc-light  the  size  of  a  pin's  head  cannot  be  examined 
without  the  aid  of  dark  glasses,  it  is  so  overwhelmingly  bright.  And  its  tempera- 
ture is  6,300°  F.  But  this  star  is  nearly  a  million  miles  through  and  is  very 
much  brighter  and  hotter  ! 


60     HOW  TO   KNOW  THE   STARRY  HEAVENS 

As  we  watch  this  stormy  scene  it  reminds  us  of  a  wind-tossed 
prairie  fire  as  seen  by  night  through  a  telescope  on  our  little 
Earth.  The  flames  rise  and  dart  forward,  fall  back  and  roll 
over ;  bend,  twist,  and  curl ;  embrace,  wrestle,  and  fling  them- 
selves apart.  Before  our  eyes  they  change  into  all  imaginable 
shapes,  so  that  we  find  it  almost  impossible  to  realise  their  over- 
whelming magnitudes  and  the  terrific  speed  of  their  varied 
movements.  Every  once  in  a  while  we  see  great  ruddy  blasts 
of  fiery  gas  rise  from  the  surface  with  tremendous  force  and  in- 
conceivable velocity.  Some  of  these  flaming  jets  shoot  up  at 
the  rate  of  250  miles  in  a  second  of  time,  and  reach  an  altitude 
of  200,000  or  300,000  miles.  They  then  branch  out  in  tree-like 
clouds,  and  finally  break  up  and  scatter  in  a  shower  of  solar 
fireworks.  The  very  largest  of  these  flames  are  long  enough  to 
be  wrapped  sixteen  times  around  our  Earth. 

These  ruddy  flames  (though  cooler  than  the  white  photosphere 
beneath  them)  are  so  inconceivably  hot  that  they  do  not  burn. 
In  fact  they  would  "  unburn  "  any  burnt  substance  which  might 
fall  into  them,  even  if  it  should  happen  to  be  as  large  and  heavy 
as  our  Earth.  No  chemical  compound  could  exist  for  a  second 
in  such  a  terrific  heat.  No  element,  even,  could  remain  there  in 
a  solid  or  liquid  state.  The  flames  are  composed  of  incandescent 
hydrogen  and  helium,  while  the  ruddy  sea  from  which  they  rise 
contains  also  iron,  magnesium,  sodium,  and  other  metals,  all 
vaporised  by  the  tremendous  heat. 

This  scarlet  ocean  of  fiery  gas  is  termed  a  sierra  or  chromo- 
sphere. The  flames  which  rise  from  it  are  known  as  prom- 
inences. 

Outside  of  all  these  is  a  corona,  consisting  of  great  hazy  radi- 
ating streaks  of  some  light  and  apparently  gaseous  substance, 
extending  a  million  miles  or  more  into  outer  space.  Owing  to 
their  distribution,  and  to  the  fact  that  the  star  is  spinning 
around  on  its  axis,  these  "  repulsive  "  streaks  are  not  straight, 
but  slightly  curved,  and  have  a  very  peculiar  plume-like 
appearance. 


FIG.  28.  —  SOLAR  CORONA.    ECLIPSE  OF  MAY  28,    1900 
Photographed  by  Chabot-Dolbeer  Eclipse  Expedition. 


FIG.   29.  —  XOKTH   POLAR   STREAMERS  OF  THE  CORONA.     MAY  28,    1900 
Crocker  Eclipse  Expedition. 


THE  CHARIOT  OF  IMAGINATION  61 

OFFSPRING  OF  A  STAR 

As  we  watch  the  eruptive  "  freckles  "  which  come  and  go  every 
eleven  years  on  the  surface  of  the  star,  we  notice  a  number  of 
small  balls  sweeping  around  and  around  it,  all  going  in  the  same 
general  direction.  These  are  all  worlds,  more  or  less  like  the 
one  on  which  we  used  to  live  before  we  began  our  heavenly 
wanderings.  Let  us  watch  them  as  they  eddy  around  the  star, 
like  moths  circling  around  a  lantern  in  the  dark. 

THREE  CLASSES  OF  WORLDS 

The  most  noticeable  of  them  are  four  outer  or  superior  planets. 
These  are  so  much  larger  and  more  powerful  than  the  rest  that 
they  form  a  kind  of  aristocracy,  subject  only  to  the  reigning 
monarch  in  the  centre.  They  do  not  appear  to  shine  by  their 
own  light,  yet  they  are  still  puffed  up  with  heat.  They  have 
followers,  or  satellites,  of  their  own,  so  that  they  are  something 
like  petty  rulers  subject  to  a  higher  power. 

Four  very  insignificant  planets  form  an  inner  or  inferior  family 
of  worlds.  They  give  out  neither  light  nor  heat  of  their  own, 
so  they  may  be  called  terrestrial  planets.  They  are  much  more 
under  the  control  of  the  central  ruler,  but  at  the  same  time  may 
be  said  to  bask  in  the  sunshine  of  his  favour.  They  are,  in  fact, 
the  bourgeoisie  or  well-to-do  citizens  of  the  monarchy. 

Between  these  two  families  of  worlds  there  is  a  whole  regiment 
of  almost  invisible  planets,  which  may  be  called  asteroids,  from 
their  small  size  and  star-like  appearance.  They  are  the  pro- 
letarians, the  working-class  of  the  monarchy,  subject  not  only  to 
the  legitimate  rule  of  the  sovereign,  but  also  to  the  overbearing 
authority  of  the  aristocracy  on  the  one  hand,  and  to  the  petty 
bossing  of  the  bourgeoisie  on  the  other.  They  are  in  fact  "  be- 
tween the  upper  and  the  nether  millstone."  The  result  is  shown 
by  the  steep,  elongated,  and  apparently  dangerous  paths  some  of 
them  are  compelled  to  follow,  with  neither  hope  of  relief  nor 
promise  of  reward. 


62     HOW   TO   KNOW   THE   STARRY   HEAVENS 

Although  the  four  outer  planets  appear  to  us  to  be  very  large, 
yet  they  are  extremely  small  compared  with  the  central  star, 
which  is  560  times  as  large  and  745  times  as  heavy  as  all  the 
planets  put  together. 

Let  us  now  examine  some  of  the  individuals  composing  these 
three  classes  of  worlds,  beginning  with  the  inner  planets. 

THE  INNER  PLANETS 

The  one  which  is  nearest  to  the  central  star  is  small,  and  very 
little  can  be  seen  of  it.  The  second  is  larger,  with  a  dense  atmos- 
phere which  somewhat  obscures  the  planet  itself.  Both  of  these 
little  wgrlds  move  at  a  speed  many  times  greater  than  that  of  a 
cannon-ball,  yet  it  takes  them  several  months  to  go  once  around 
the  central  star. 

PLANET  NUMBER  THREE 

Planet  No.  3  is  slightly  larger  than  No.  2,  its  diameter  being 
nearly  8,000  miles.  It  is,  indeed,  the  largest  of  the  inner  family 
of  worlds.  It  is  more  than  90,000,000  miles  from  the  star 
around  which  it  is  sweeping.  It  takes  just  a  year  to  complete  a 
revolution,  although  it  travels  at  the  astounding  rate  of  eighteen 
miles  in  a  second  of  time. 

On  looking  more  closely  at  this  world,  another  and  smaller 
planet  is  seen  buzzing  around  and  around  it.  This  is  a  moon  or 
satellite,  which  goes  around  its  primary  in  the  same  direction  as 
that  is  going  around  the  central  star.  It  is  a  little  over  2,000 
miles  thick,  so  that  the  principal  planet  is  about  fifty  times  as 
large  as  its  companion. 

A  more  attentive  look  at  No.  3  reveals  several  peculiarities. 
It  is  spinning  around  like  a  top,  turning  in  the  same  gen- 
eral direction  as  that  in  which  it  goes  around  the  central  star. 
The  two  points  which  form  its  poles  of  rotation  are  white,  as 
though  covered  with  ice  and  snow.  Its  surface  is  variegated, 
and  is  evidently  composed  of  land  and  water.  There  are  con- 
tinents, oceans,  islands,  seas,  lakes,  rivers,  and  mountains.  The 
land  appears  to  be  more  or  less  covered  by  vegetation  of  a  green 


FIG.   30.  —  MERCURY-,   THE  FIRST  PLANET 
By  Schiaparelli.    (From  Todd's  "  Stars  and  Telescopes,"  published  by  Messrs.  Little,  Brown,  <fe  Co. ) 


FIG.  31.  —  VENUS,  THE  SECOND  PLANET 

By  Autoniadi.     (From  Comstock's  "  Text-book  of  Astronomy,"  published  by 
Messrs.  D.  Appleton  &  Co.) 


FIG.   33.  —  MARS,   THE  FOURTH  PLANET 

By  Knobel.     (From  Todd's  "  Stars  and  Telescopes," published  by  Messrs.  Little,  Brown,  &  Co.) 


THE   CHARIOT  OF  IMAGINATION  63 

colour,  while  portions  of  the  surface  are  hidden  by  drifting 
clouds.  Altogether,  it  looks  like  a  world  which  might  be  in- 
habited. Although  small  compared  with  the  giant  planets 
which  circle  on  the  outskirts  of  the  system,  yet  the  diversity  of 
its  surface,  and  the  terrific  speed  with  which  it  circles  around 


FIG.  32.  —  TERRA,  THE  THIRD  PLANET,  AND  ITS  SATELLITE 
OR  MOON 

the  central  star,  entitle  it  to  the  archangel's  song  as  given  in  the 
prologue  to  Goethe's  "  Faust " : 

"  And  swift  and  swift  beyond  conceiving, 
The  splendour  of  the  World  goes  round, 
Day's  Eden-brightness  still  relieving 
The  awful  night's  intense  profound. 
The  ocean  tides  in  foam  are  breaking, 
Against  the  rock's  deep  bases  hurled, 
And  both,  the  spheric  race  partaking, 
Eternal,  swift,  are  onward  whirled." 

—  Bayard  Taylor's  Translation. 

PLANET  NUMBER  FOUR 

Planet  No.  4  has  a  ruddy  appearance.  It  is  considerably 
smaller  than  the  one  just  described,  its  diameter  being  only 
about  4,000  miles.  It  has  two  very  small  moons  circling  around 
it.  Like  No.  3,  it  has  the  appearance  of  being  in  a  condition 
suitable  for  sustaining  life.  It  appears  to  have  an  atmosphere, 
clouds,  and  variegated  continents.  Its  poles,  too,  are  white,  as 
though  covered  by  ice  and  snow. 


64     HOW  TO   KNOW  THE   STARRY   HEAVENS 


PLANETOIDS 

After  Planet  No.  4  there  is  a  great  crowd  of  little  worlds 
which  are  too  small  to  be  of  much  importance.  The  most  in- 
teresting thing  about  them  is  the  speculation  whether  or  not 

they  are  the  fragments  of 
two  larger  planets  that 
have  unsuccessfully  tried 
to  occupy  the  same  space 
MA  us  at  the  same  time. 

A  GIANT  PLANET 

Outside  this  swarm  of 
pigmy  worlds  there  circles 
and  spins  a  gigantic  ball 
about  1,200  times  the  size 
of  Planet  No.  3.  It  is  not 
only  the  largest  of  all  the 
planets,  but  is  larger  than 
all  •  the  rest  of  them  put 
together.  It  has  a  dense 
impenetrable  atmosphere, 
with  bands  of  clouds  around 
its  equatorial  regions. 
There  are  five  moons  sweep- 
ing around  it,  some  of  them 
being  of  considerable  size, 
far  larger  than  any  of  the 
planetoids  just  mentioned. 
The  main  planet,  although 
so  large,  spins  around  in  a  little  less  than  ten  hours.  This  rapid 
rotation  has  made  its  equator  bulge  out,  so  that  the  cloud-like 
surface  of  the  planet  is  something  the  shape  of  an  orange. 

A  RINGED  PLANET 

After  this  there  is  another  big  globe,  only  second  in  size  to 
the  one  just  described.  But  this  one  is  surrounded  by  such  an 


FIG.  34.  —  RELATIVE  SIZES  OF  EARTH 
AND  MARS 


FIG.   35.  —  THE  ZONE  OF  ASTEROIDS  BETWEEN  MARS  AND  JUPITEU 


FIG.   36.  —  JUPITER,   THE   LARGEST   I'LANET 
Showing  great  red  spot  and  transit  of  satellite.     Lick  Observatory. 


THE  CHARIOT  OF  IMAGINATION 


65 


astonishing  arrangement  that  one  cannot  help  rubbing  his  eyes 
to  see  if  they  have  not  deceived  him.  It  looks  as  though  there 
was  a  large  round  thin  disc,  with  a  good-sized  round  hole  in  the 
middle.  In  the  centre  of  this  hole  is  the  planet  itself,  not  quite 
large  enough  to  fill  the  hole,  and  not  visibly  connected  with 
the  disc.  On  a  closer  view  the  disc  is  seen  to  have  a  series  of 
gaps  and  thin  places  in  it, 
extending  all  around,  as 
though  it  were  really  a 
series  of  discs  all  lying  in 
the  same  plane,  and  hav- 
ing the  planet  for  a  centre, 
but  differing  in  size  and 
texture.  If  these  concen- 
tric discs  or  rings  were 
visibly  supported  by  the 
planet,  they  would  not 
seem  so  extraordinary, 
but  the  closest  scrutiny 
fails  to  discover  any  phy- 
sical connection  with  the 
globe  they  surround.  The 
only  explanation  that 
seems  able  to  account  for 
their  continued  existence 
is  that  they  are  composed 
of  countless  millions  of 
tiny  satellites  crowded 

together,  and  revolving  around  the  planet  in  the  same  direction.1 
Farther  off  than  these  rings  there  are  nine  satellites,  or 
moons,  revolving  at  different  distances  from  the  planet.  The 
whole  family  group  seems  almost  a  miniature  of  the  solar  sys- 
tem of  which  it  forms  a  part,  with  a  hint  thrown  in  as  to  how 
that  system  originated. 

1  If  they  were  farther  from  the  planet,  they  would  probably  coalesce  into 
regular  satellites,  but  as  it  is,  the  tidal  action  of  the  huge  planet  prevents 
this.  5 


FIG.  37. —RELATIVE  SIZES  OF  JUPITER 
AND  EAKTH 


66     HOW  TO   KNOW  THE   STARRY   HEAVENS 


TWO  OUTER  PLANETS 

There  are  two  more  large  planets  outside  the  one  just  described. 
But  they  are  not  so  remarkable  in  appearance,  and  they  are  so  far 
from  the  central  star  that  they  are  comparatively  in  the  cold  and 
dark.  The  outside  one  is  about  30  times  as  far  off  as  Planet 
No.  3,  and  is  125  times  as  large.  It  moves  only  a  little 
over  three  miles  in  a  second  of  time,  and,  as  it  has  a  large 
circuit  to  make,  it  takes  164  years  to  go  once  around  the 

central  star. 

It  is  interesting  to 
note  that  the  combined 
mass  of  the  four  out- 
side planets  just  men- 
tioned is  about  220 
times  as  great  as  the 
combined  mass  of  the 
four  inner  planets,  with 
.all  the  visible  planet- 
oids thrown  in.  Also 
that  the  same  outside 
planets  together  weigh 
450  times  as  much  as 
No.  3  alone.  If  only 
bulk  and  mass  were 

concerned,  it  would  'scarcely  be  worth  while  to  mention  the 
inner  planets  at  all,  they  are  so  insignificant. 

SOLAR  SYSTEM  No.  3,141,592,653 

It  is  hardly  necessary  to  explain  that  the  star  we  have  been 
examining,  with  its  attendant  worlds,  forms  our  own  Solar  Sys- 
tem. The  insignificant  little  globe  which  I  have  called  "  Planet 
No.  3  "  is  our  own  World,  once  regarded,  by  its  reasoning  inhab- 
itants, as  the  whole  Universe,  with  nothing  outside  but  the 
Realms  of  Chaos. 

If  we  were  to  approach  any  one  of  the  millions  upon  millions 
of  sovereign  suns  revealed  by  the  telescope,  we  should  in  all 


FIG. 


).— RELATIVE  SIZES  OF  SATURN 
AND  EARTH 


H 
W 

I  I 


THE   CHARIOT   OF   IMAGINATION 


67 


probability  find  it  to  be  the  centre  of  a  family  group  more  or 
less  similar  to  ours.  Some  systems  are  smaller,  but  others  are 
far  larger,  while  many  are  more  elaborate  and  democratic,  with 
two,  three,  four,  a  hundred,  or  a  thousand  suns  circling  around 
their  common  centre  of  gravity. 


FIG.  40.  — RELATIVE  SIZES  OF  NEPTUNE 
AND  EARTH 


CHAPTER  VI 

DIMENSIONS  OE  THE  UNIVERSE 

"Firstly,  we  may  inquire  as  to  the  extent  of  the  Universe  of  stars.  Are  the 
latter  scattered  through  infinite  space,  so  that  those  we  see  are  merely  that  por- 
tion of  an  infinite  collection  which  happens  to  be  within  reach  of  our  telescopes, 
or  are  all  the  stars  contained  within  a  certain  limited  space  ? " 

—  Prof.  Simon  Newcorrib. 

PLANETARY  DISTANCES 

IN  the  third  and  fourth  chapters  I  tried  to  show  the  principles 
by  which  the  distances  of  the  heavenly  bodies  are  measured. 
It  was  there  stated  that  the  Moon's  distance  from  us  is  about 
239,000  miles,  and  that  the  Sun  is  about  388  times  as  far  off. 
The  most  reliable  measurements  make  the  Earth's  distance  from 
the  Sun  92,790,000  miles.  Neptune,  the  farthest  planet  in  our 
system,  is  about  30  times  as  far  from  the  Sun  as  we  are,  so  that 
it  is  2,790,000,000  miles  away. 

This  distance  is  so  tremendous  that  it  is  unthinkable.  The 
mind  of  man  cannot  grasp  it.  It  is  as  utterly  beyond  our  com- 
prehension as  infinity  itself.  Yet  when  we  turn  to  the  stars  we 
find  that  this  vast  distance  is  as  nothing  when  compared  to  the 
intervals  separating  one  star  from  another. 

STELLAR  DISTANCES 

It  has  been  ascertained  that  the  nearest  star  outside  our  sys- 
tem is  something  like  9,000  times  as  far  off  as  Neptune,  whose 
distance  seemed  so  amazingly  great.  Alpha  Centauri,  the  nearest 
of  all  the  stars,  is  distant  from  us  about  25,000,000,000,000 
miles.  Its  light,  travelling  at  the  rate  of  186,000  miles  in  a 
second  of  time,  takes  more  than  four  years  to  come  to  us. 

The  brightest  star  in  all  the  sky  is  known  as  Sirius,  or  the 
Dog  Star.  It  is  twice  as  far  from  us  as  Alpha  Centauri,  and  is 
therefore  more  than  eight  "light-years "  away. 


DIMENSIONS   OF  THE   UNIVERSE  69 

The  distances  of  about  sixty  other  stars,  ranging  up  to  sixty 
light-years,  have  been  more  or  less  approximately  ascertained. 
All  the  others  are  too  far  off  for  our  sounding-rods.  They  are  out 
of  reach  in  the  depths  of  space.  All  that  we  know  for  certain 
concerning  their  distances  is  that  none  of  them  are  less  than 
4,000,000  times  as  far  from  us  as  our  Sun,  which  is  nearly 
93,000,000  miles  away.1 

COMPARISONS 

Now  it  is  very  easy  to  say  that  the  nearest  star  outside  our 
system  is  25,000,000,000,000  miles  away,  but  it  is  not  so  easy 
to  realise  what  that  distance  is  like.  Let  us  try  to  do  so  by 
means  of  some  simple  illustrations. 

In  the  first  place  it  is  necessary  to  realise  the  proportionate 
distances  and  dimensions  of  the  members  of  our  own  Solar 
System. 

If  we  take  a  one-inch  ball  to  represent  our  Earth,  it  will 
require  a  nine-foot  globe  to  represent  the  Sun. 

Let  us  place  this  nine-foot  globe  on  a  level  plain  that  has 
just  had  the  grass  burnt  off  it,  and  set  up  wire  circles  (on  posts) 
to  indicate  the  various  planetary  orbits. 

The  sizes  of  the  planets  and  the  distances  of  their  orbits  from 
the  central  globe  will  be  as  follows : 

PLANET 

Mercury  . 
Venus 

Earth      .     . 
Mars       .     . 

Asteroids 

Jupiter    .     . 
Saturn     . 
Uranus     . 
Neptune  . 

1  Investigations  are  now  in  progress  which  promise  greatly  to  extend  the  list 
of  stars  having  a  measurable  parallax.  It  is  possible  that  the  distances  of  most  of 
the  naked-eye,  stars  will  be  ascertained  before  many  years  have  passed. 


SIZE 

DISTANCE 

large  pea  . 

127  yards 

one-inch  ball 

235     " 

one-inch  ball 

325     " 

half-inch  marble  . 

495     " 

small  seeds 

676     " 
1,385     " 

eleven-inch  globe 

1    mile  (nearly) 

nine-inch  globe   . 

If  miles 

four-inch  globe    . 

3£     " 

five-inch  globe     . 

5£     '< 

70     HOW  TO   KNOW  THE   STARRY   HEAVENS 

On  this  scale  our  Moon  will  be  represented  by  a  pea  moving 
in  a  circle  at  a  distance  of  30  inches  from  the  one-inch  ball 
representing  the  Earth. 

The  outside  ring  of  Saturn  will  be  21  inches  in  diameter. 

The  orbit  of  Neptune  will  be  11  miles  across,  and  will  take 
nearly  35  miles  of  wire  to  mark  it  out. 

Let  us  now  make  arrangements  with  an  electric  light  com- 
pany to  cover  our  nine- foot  globe  with  a  complete  network  of 
electric  lights,  so  close  and  compact  that  every  part  of  the  sur- 
face will  give  off  more  light  and  heat  than  the  brightest  part  of 
an  arc-light.  We  will  then  choose  a  dark  yet  clear  night,  and 
turn  on  the  current. 

We  shall  find  that  all  of  our  toy  planets  are  more  or  less  bril- 
liantly illuminated  on  one  side  by  the  central  light.  But  of 
course  the  outer  side  of  each  "  planet "  will  be  dark  and  invis- 
ible. Let  us  station  ourselves  just  beneath  the  one-inch  ball 
which  represents  our  Earth,  and  take  a  look  around  at  our 
miniature  "solar  system." 

The  overpowering  effulgence  of  the  nine-foot  globe,  325  yards 
away,  makes  it  necessary  for  us  to  put  up  a  shelter  to  protect  us 
from  the  light  and  heat. 

Apart  from  our  electric  "  sun,"  the  most  prominent  object 
visible  from  our  position  will  be  the  quarter-inch  ball  which  goes 
around  us  at  a  distance  of  30  inches.  It  will,  in  fact,  look  as  large 
as  our  nine-foot  "  sun,"  but  will  not  be  anything  like  as  bright. 

As  the  illuminated  side  of  this  imitation  "  moon "  does  not 
always  face  us,  it  will  show  all  the  phases  of  the  real  Moon  as 
it  goes  around  in  its  little  orbit. 

The  first  of  our  toy  planets  (that  is,  the  one  nearest  to  the 
central  globe)  will  be  just  visible  to  us  when  most  favourably 
placed.  The  second  will  shine  very  brightly  when  it  is  at  a 
large  angle  from  our  "sun."  Both  of  these  toy  planets  will 
show  lunar  phases  if  examined  with  the  help  of  a  telescope. 

The  fourth  "  planet "  (half  an  inch  in  diameter),  will  also  be 
very  brilliant  when  at  its  nearest,  170  yards  away.  But  at 
other  times  it  will  be  rather  inconspicuous. 


FIG.   41.  —  LICK  OBSERVATORY   ON  MOUNT  HAMILTON,    CALIFORNIA 
FIG.  42.  —  MAIN  ENTRANCE  AND  GREAT  DOME,   LICK  OBSERVATORY 


DIMENSIONS   OF  THE   UNIVERSE  71 

The  first  and  second  of  the  large  planets  will  also  be  tolerably 
conspicuous,  although  a  telescope  will  be  necessary  to  show 
details. 

The  two  outside  planets  will  not  be  visible  at  all  without  a 
telescope.  The  farthest  of  them  will  not  vary  much  in  appear- 
ance ;  its  distance  at  all  parts  of  its  orbit  being  something  like 
5  J  miles  away  from  us. 

The  nearest  star  will,  on  the  same  scale,  be  represented  by  an 
electric  globe,  probably  12  feet  across,  at  a  distance  of  about 
50,000  miles.  Sirius  will  take  a  similar  globe,  nearly  30  feet  in 
diameter,  about  100,000  miles  away.  The  largest  star  visible 
to  us  may  probably  be  represented  by  a  100-foot  globe,  some- 
where about  1,000,000  miles  distant. 

This  illustration  gives  a  fair  idea  of  the  comparative  dimen- 
sions and  distances  of  the  principal  bodies  forming  our  Solar 
System,  but  fails  to  convey  any  definite  idea  of  stellar  distances. 
Let  us  try  some  other  illustrations. 

A  LOCOMOTIVE 

We  all  know  what  it  is  to  travel  at  the  rate  of  60  miles  an 
hour.  At  this  rate  it  would  take  17 J  days  and  nights  to  travel 
around  our  own  world. 

To  reach  the  Moon,  travelling  at  the  same  rate,  it  would  take 
166  days,  or  nearly  half  a  year. 

To  reach  the  Sun,  we  should  have  to  travel  for  176  years. 

To  go  around  the  Sun  would  take  about  5  years. 

To  go  from  the  Sun  to  Neptune,  the  farthest  planet,  would 
take  5,000  years,  and  the  railway  fare,  at  one  cent  a  mile,  would 
be  nearly  $28,000,000.  This  makes  a  railroad  impracticable. 

A  CANNON-BALL 

Now  take  a  cannon-ball  travelling  at  the  rate  of  a  mile  in  five 
seconds. 

It  would  go  around  the  Earth  in  36  hours,  and  would  reach 
the  Moon  in  14  days. 


72     HOW  TO   KNOW  THE   STARRY   HEAVENS 

To  get  to  the  Sun  would  take  it  1 5  years,  and  it  would  require 
5  months  to  go  around  it.  To  go  from  the  Sun  to  Neptune,  the 
farthest  planet,  it  would  have  to  travel  at  the  same  speed  for 
415  years. 

AN  ARROW  OF  LIGHT 

A  wave  of  light  travels  at  the  enormous  velocity  of  186,000 
miles  in  one  second  of  time.  So  it  would  go  around  our  Earth 
more  than  seven  times  in  a  second.  It  would  reach  the  Moon 
in  a  second  and  a  quarter. 

It  takes  more  than  eight  minutes  for  the  Sun's  light  to  reach 
us,  and  over  four  hours  for  it  to  get  to  Neptune,  the  outside 
planet  in  our  own  system. 

At  the  same  rate,  186,000  miles  a  second,  it  takes  more  than 
four  years  for  the  light  of  the  nearest  star  to  reach  us. 

Sirius,  the  brightest  star  in  the  sky,  is  so  far  off  that  the  light 
which  reaches  us  to-night  has  been  eight  and  a  half  years  on  the 
way.  If  it  were  to  collide  with  another  star  now,  we  should  not 
see  the  flare-up  for  eight  years  and  a  half.1 

A  PILE  OF  PAPER 

Now,  suppose  that  we  had  here  a  pile  of  thin  paper,  with  as 
many  sheets  in  it  as  there  are  miles  between  us  and  the  nearest 
star.  What  would  be  the  height  of  the  pile,  supposing  that  it 
took  200  sheets  to  measure  one  inch  in  thickness  ? 

Those  people  who  are  naturally  reasonable  in  their  ideas  and 
moderate  in  their  estimates  might  think  that  the  pile  would 
probably  be  a  hundred  feet  or  so  in  thickness.  Others,  who  are 
naturally  wild  in  their  ideas  and  extravagant  in  their  guesses, 
would  think  that  the  pile  might  possibly  be  a  mile  thick.  As  a 
matter  of  fact,  it  would  reach  up  nearly  2,000,000  miles.  More 
exactly,  its  height  would  be  1,972,853  miles,  94%  yards,  and  8 
inches. 

1  The  distance  of  Sirius  is  not  less  than  that  stated,  but  there  is  a  possibility 
that  it  is  greater. 


FIG.   43.  —  THE  THIRTY-SIX-IXCH  REFRACTOR  AT  LICK   OBSERVATORY 

The  tube  is  5G  feet  long  and  weighs  several  tons.     It  is  equatorially  mounted,  with  a 

driving  clock.     The  entire  floor  under  the  dome  can  be  raised  or 

lowered  26  feet.     It  is  shown  half-way  down. 


DIMENSIONS   OF  THE   UNIVERSE  75 

As  this  pile  of  paper  is  rather  top-heavy,  suppose  we  lay  it 
down  on  its  side.  Then  it  will  go  nearly  79  times  round  the 
Earth.  Yet  every  inch  in  this  great  pile  of  paper  represents  a 
distance  of  200  miles.  The  sheets  would  have  to  be  placed  a 
mile  apart  before  they  would  reach  to  the  nearest  star. 

A  STACK  OF  BLOOD  DISCS 

Perhaps  a  smaller  illustration  of  the  same  kind  may  be  more 
within  our  mental  grasp. 

Human  blood  is  an  almost  colourless  fluid,  crowded  with  very 
small  red  discs  or  corpuscles.  These  discs  are  flat  and  coin- 
shaped.  They  are  so  extremely  minute  that  if  they  were  piled 
up  one  on  the  top  of  another,  like  a  stack  of  coins,  it  would  take 
15,000  of  them  to  reach  one  inch  in  height. 

If  we  let  each  disc  stand  for  one  mile,  then  the  height  of  the 
pile  representing  the  Moon's  distance  will  be  16  inches.  That 
representing  the  Sun's  distance  will  reach  up  172  yards,  and 
Neptune's  pile  will  be  nearly  three  miles  high.  But  the  pile 
which  contains  as  many  discs  as  there  are  miles  between  us  and 
the  nearest  star  will  be  26,000  miles  in  height.  If  it  were  laid 
down  on  the  ground  it  would  go  around  the  world. 

VIOLET  WAVES 

The  shortest  light-waves  which  affect  the  eye  are  those  which 
produce  the  sensation  of  violet  light.  It  takes  61,000  of  these 
waves  to  measure  one  inch.  If  a  single  wave  stands  for  a  mile, 
then  the  distance  of  the  Moon  will  be  represented  by  4  inches ; 
of  the  Sun,  by  42  yards ;  of  Neptune,  by  1,250  yards ;  and  of 
Alpha  Centauri,  by  6,400  miles. 

Then  we  must  remember  that  the  vast  majority  of  even  naked- 
eye  stars  are  hundreds  and  thousands  of  times  farther  off  than 
the  one  we  have  been  considering. 

A  LONG  STRAND 

The  cotton  factories  of  Lancashire,  England,  at  present  spin 
about  155,000,000  miles  of  thread  in  a  day,  so  that  in  six  seconds 


74     HOW  TO   KNOW  THE  STARRY  HEAVENS 

they  make  enough  to  go  around  the  Earth.  In  one  minute  they 
spin  enough  to  reach  from  here  to  the  Moon.  The  product  of 
18  days  would  reach  from  the  Sun  to  Neptune.  Counting  310 
working  days  in  a  year,  it  would  take  them,  at  this  rate,  500 
years  to  spin  enough  thread  to  reach  to  the  nearest  star. 

If  one  end  of  this  thread  were  to  be  made  fast  to  some  place 
on  our  equator,  the  daily  rotation  of  the  Earth  would  wind  up 
25,000  miles  of  it  every  day.  At  this  rate  it  would  take  about 
300  years  to  wind  up  that  part  of  the  thread  between  us  and 
Neptune.  But  to  wind  up  the  whole  of  the  thread  between  us 
and  the  nearest  star  would  take  £,500,000  years. 

Let  us  suppose  that  this  thread  is  all  wound  around  the  Earth, 
and  that  the  size  of  the  thread  is  such  that  a  rope  of  it  an  inch  in 
diameter  contains  10,000  threads.  Then  the  entire  skein  would 
make  a  rope  nearly  26  feet  in  diameter  around  the  entire  Earth. 

Let  us  suppose  that  four  miles  of  this  thread  weighs  one 
pound.  Then  that  part  of  it  between  the  Sun  and  Neptune  will 
weigh  340,000  American  tons.  And  the  same-sized  thread 
reaching  to  the  nearest  star  will  weigh  -3,000,000,000  American 
tons.  If  twenty  tons  of  it  were  to  be  loaded  on  one  railroad  car,  it 
would  take  17,000  cars  to  carry  that  part  between  the  Sun  and 
Neptune,  and  the  thread  reaching  to  the  nearest  star  would  take 
150,000,000  cars  to  hold  it  all. 

A  SPIDER'S  THREAD 

These  numbers  are  still  too  large  to  be  realised  by  the  human 
mind  as  at  present  constituted.  Fortunately,  however,  there  is 
a  way  of  considerably  reducing  them. 

The  thread  spun  by  some  spiders  is  so  extremely  fine  and  light 
that  a  single  pound  of  it  would  be  long  enough  to  reach  around 
our  Eai\h.  Let  us  see  if  we  cannot  get  more  reasonable  weights 
by  using  this  light  and  invisible  thread. 

To  reach  to  our  Moon  would  require  nearly  10  pounds  of  it. 
To  go  to  the  Sun  would  take  3,712  pounds.  Neptune's  distance 
would  require  56  American  tons  of  it. 


FIG.  44.  —  EYE-PIECE  OF  THE  GREAT  LICK  TELESCOPE 


DIMENSIONS  OF  THE   UNIVERSE  75 

But  to  reach  to  the  nearest  star  would  take  500,000  tons. 
At  twenty  tons  to  the  car,  this  would  take  25,000  cars  to  carry 
it  all.  At  35  feet  to  the  car,  these  would  make  a  train  of  cars 
167  miles  in  length.  This  train  would  require  500  powerful 
locomotives  to  move  it. 

A  QUARTZ  FIBRE 

We  can  get  still  better  results  by  taking  a  quartz  fibre  like 
those  used  in  a  torsion-balance  for  weighing  the  Earth.  They 
can  be  made  one-hundred-thousandth  of  an  inch  in  diameter, 
with  tapering  ends  which  thin  off  to  the  millionth  of  an  inch. 
No  microscope  can  show  a  fibre  of  this  latter  size,  but  its  presence 
can  be  made  apparent  by  means  of  photography.  Nine  and  a 
half  grains  of  this  invisible  quartz  fibre  (equal  to  one  seventieth 
of  a  cubic  inch)  would  reach  to  the  Moon.  Half  a  pound  of  it 
would  go  nearly  to  the  Sun.  Sixteen  pounds  would  reach  to 
Neptune.  But  it  would  take  72  American  tons  (equal  to  a  cube 
of  9  feet  4  inches)  to  go  to  Alpha  Centauri. 

ONE  CENT  A  MILE 

If  a  railroad  could  be  constructed  to  the  nearest  star,  and  the 
fare  made  one  cent  a  mile,  a  single  passage  would  cost  $860000- 
000,000.  This  would  make  a  94-foot  cube  of  pure  gold.  The 
coined  gold  in  the  world  amounts  to  $4,000,000,000,  about  equal 
to  a  24-foot  cube.  It  would  therefore  take  more  than  60  times 
the  world's  stock  of  coined  gold  to  pay  the  fare  of  one  passenger. 
Let  us  save  up  our  money  and  go  when  the  line  is  built ! 

A  TIRELESS  WHEEL 

One  more  illustration  will  bring  this  chapter  to  a  close. 

Let  us  suppose  a  wheel  to  be  turned  at  the  speed  of  100  revolu- 
tions in  a  second  of  time.  This  is  equal  to  6,000  times  in  a  minute. 
At  that  speed  it  will  go  around  1,000,000  times  in  a  little  less 
than  three  hours.  To  go  around  as  many  times  as  there  are 
miles  between  us  and  the  Sun  would  take  nearly  eleven  days. 
To  go  around  as  many  times  as  there  are  miles  between  the  Sun 


76     HOW  TO  KNOW  THE  STARRY  HEAVENS 

and  Neptune,  the  outside  planet,  would  take  nearly  eleven 
months.  But  to  go  around  as  many  times  as  there  are  miles 
between  us  and  the  nearest  star  outside  our  system  would  take 
nearly  8,000  years. 

These  comparisons  will  do  for  the  present.    Let  us  take  a  rest 
—  till  the  next  chapter. 


CHAPTER  VII 

SOME  MORE  DIMENSIONS 

"  That  collection  of  stars  which  we  call  the  UnfVerse  is  limited  in  extent. 
.  .  .  This  does  not  preclude  the  possibility  that  far  outside  of  our  Universe  there 
may  be  other  collections  of  stars  of  which  we  know  nothing." 

—  Prof.  Simon  Newcomb. 

"  Then  the  angel  threw  up  his  glorious  hands  to  the  Heaven  of  Heavens,  say- 
ing :  '  End  is  there  none  to  the  Universe  of  God.  Lo,  also,  there  is  no  begin- 
ning ! '  "  —  De  Quincey. 

A  LONG  SHEET  OF  PAPER 

AYEEY  good  way  to  get  an  idea  of  the  relative  distances 
of  the  heavenly  bodies  is  to  mark  them  off,  on  a  small 
scale,  on  a  long  sheet  of  paper. 

In  order  to  do  this  properly,  get  a  1,000-pound  roll  of  paper, 
like  that  on  which  magazines  are  printed.1  Set  up  a  bench  or 
table,  about  4  miles  long,  and  unroll  the  paper  on  it.  Draw 
a  straight  line  down  the  middle  of  it,  from  one  end  to  the 
other.  We  can  now  choose  a  unit  of  measure,  and  mark  off 
the  distances  along  this  line. 

Let  us  make  a  mark  to  represent  the  Sun,  and  another,  one 
inch  away,  to  represent  the  Earth.  Then  one  inch  will  stand 
for  93,000,000  miles.  On  the  same  scale,  Mars,  the  ruddy 
planet,  will  be  \\  inches  off;  Jupiter,  the  largest  of  the  planets, 
will  be  9^  inches  distant;  Uranus  will  be  19  inches  off;  and 
Neptune,  the  farthest  of  all  the  planets,  will  be  30  inches 
away.2  The  distance  that  light  will  travel  in  one  year  will  be 

1  Such  a  roll  is  20,250  feet  long  and  39  inches  wide. 

2  The  orbits  of  all  the  principal  planets  are  on  nearly  the  same  plane.     There- 
fore, on  the  above  scale,  the  Solar  System  could  be  contained  inside  a  round  disc 
of  wood  five  feet  in  diameter  and  two  inches  thick.     Very  few  even  of  the  small 
planets  (asteroids)  would  ever  go  outside  of  this  thin  disc. 


78     HOW  TO   KNOW  THE   STARRY  HEAVENS 

represented  by  one  mile.  The  nearest  star,  on  the  same  scale 
of  one  inch  to  the  Sun's  distance,  will  be  more  than  four  miles 
away.  And  Sirius  will  be  eight  and  a  half  miles  distant. 

But  stay !  Our  long  sheet  of  paper  is  too  short.  Let  us 
erase  these  marks  and  try  a  smaller  scale. 

In  order  to  get  the  most  convenient  unit,  measure  off  a  12- 
inch  line  and  divide  it  into  ten  equal  parts.  Then  divide  one 
of  these  parts  also  into  ten.  This  will  give  us  the  one  hundredth 
of  a  foot  for  a  unit.  -We  may  regard  it  as  equal  to  one  eighth  of 
an  inch,  although  it  is  really  a  trifle  shorter. 

If  we  make  this  little  unit  represent  a  mile,  the  distance  of 
our  Moon  from  us  will  be  represented  by  800  yards.  It  is  quite 
evident  that  this  scale  is  too  large  for  our  purpose.  We  must 
choose  a  smaller  one. 

As  our  Moon  is  nearer  to  us  than  any  of  the  other  heavenly 
bodies,  we  will  let  our  little  unit  represent  the  Moon's  distance. 

First,  make  a  mark,  to  represent  the  Sun,  at  one  end  of  our 
long  sheet  of  paper.  Then  put  another  mark,  nearly  four  feet 
away,  to  stand  for  the  Earth,  with  a  quarter-inch  circle  around 
it  to  represent  the  Moon's  orbit.  Neptune's  place  may  now  be 
marked  out,  at  a  distance  of  117  feet  from  our  starting-point. 
The  position  of  the  nearest  star  will  be  about  200  miles  far- 
ther on. 

As  our  paper  is  again  too  short,  we  will  reduce  the  scale,  and 
regard  our  unit  as  representing  the  distance  of  the  Earth  from 
the  Sun.  Then  Neptune's  distance  will  be  3|  inches,  and  that 
of  Alpha  Centauri  will  be  900  yards. 

Even  this  small  scale  is  too  large  for  some  of  the  star-dis- 
tances which  have  been  approximately  ascertained.  So  we  will 
try  again.  This  time  we  will  represent  the  distance  of  Neptune 
from  the  Sun  by  our  unit,  so  that  the  entire  Solar  System  will 
be  about  the  size  of  a  pea.  Every  eighth  of  an  inch  will  thus 
represent  2,790,000,000  miles! 

At  last  we  have  succeeded  in  our  efforts  to  get  some  of  the 
stars  on  to  our  paper.  The  distance  of  Alpha  Centauri  will  on 
this  scale  be  only  90  feet.  Sirius,  the  brightest  star  in  the 


SOME  MORE   DIMENSIONS  79 

heavens,  will  on  the  same  scale  be  represented  by  a  microscopic 
grain  of  sand-  180  feet  away,  yet  glaring  with  such  an  over- 
whelming intensity  of  light  as  sometimes  to  throw  shadows  on 
the  invisible  point  which  represents  the  Earth. 

The  great  majority  of  the  visible  stars,  however,  on  the  above 
scale  of  one  eighth  of  an  inch  to  the  distance  of  Neptune,  will 
be  scores  and  hundreds  of  miles  away.  So  that  we  shall  have 
to  try  some  other  way  of  visibly  representing  their  distances. 

SOME  BIG  SQUARES 

If,  instead  of  using  a  straight  line  on  which  to  represent 
distances,  we  use  different-sized  squares  for  the  same  purpose, 
we  shall  perhaps  be  more  successful  in  appealing  to  the  eye. 

Take  a  square  piece  of  white  pasteboard  which  measures  a 
foot  each  way.  On  each  edge  mark  off  ten  equal  divisions. 
Then  with  pencil  and  ruler  join  the  opposite  marks,  thus 
making  a  kind  of  chess-board  with  100  equal  squares.  In  the 
same  way  divide  up  one  of  these  squares  into  100  equal  squares. 
Each  of  these  smaller  squares  will  be  nearly  one  eighth  of  an 
inch  across. 

Taking  one  of  these  tiny  squares^  as  a  unit  (1),  the  whole 
square  will  contain  one  hundred  units  (100),  and  the  entire 
pasteboard  will  be  equal  to  ten  thousand  units  (10,000). 

Now  if  each  of  these  little  squares  or  units  be  made  to  repre- 
sent one  mile,  it  will  take  a  square  nearly  five  feet  across  to 
represent  the  distance  of  the  Moon  from  the  Earth.  To  repre- 
sent the  Sun's  distance  from  us,  it  will  take  a  chess-board  meas- 
uring 96  feet  each  way.  Neptune's  distance  will  be  represented 
by  a  square  measuring  525  feet.  The  distance  of  the  nearest 
star  will  take  a  chess-board  measuring  9  J  miles  each  way,  and 
covering  90  square  miles  of  land.  It  must  be  remembered  that, 
on  this  scale,  every  eighth-of-an-inch  square  of  surface  represents 
a  mile  between  us  and  the  nearest  star. 

It  is  evident  that  we  shall  have  to  try  a  smaller  scale  if  we 
are  to  make  the  stellar  distances  visible  to  the  eye  by  means  of 
squares.  If  we  let  one  of  the  tiny  squares  represent  the  distance 


80     HOW   TO   KNOW   THE   STARRY   HEAVENS 

between  us  and  the. Sun,  then  the  distance  of  the  nearest  star 
will  be  represented  by  a  chess-board  more  than  five  feet  in 
diameter. 

If  we  reduce  the  scale  still  more,  and  make  our  unit  represent 
the  distance  of  Neptune  from  the  Sun,  then  the  distance  of 
Alpha  Centauri  will  be  represented  by  nine  tenths  of  our  12- 
inch  chess-board. 

SOME  SOLID  COMPARISONS 

In  order  to  bring  our  measures  still  more  within  the  reach  of 
the  eye,  the  little  unit  just  used  may  be  cubed.  It  will  then  be 
a  solid  block,  measuring  nearly  one  eighth  of  an  inch  every ^way. 

Kegarding  this  cube  as  the  equivalent  of  a  mile,  the  Moon's 
distance  will  be  represented  by  a  7J-inch  cube.  The  Sun's  dis- 
tance will  take  a  cube  4  feet  6  inches  every  way,  and  Neptune's 
will  take  a  14-foot  cube.  The  distance  of  Alpha  Centauri  will 
be  represented  by  a  cube  measuring  290  feet  every  way. 

A   GREAT   VOID 

Let  us  now  consider  in  other  ways  the  vastness  of  this  appar- 
ently empty  space  between  our  system  and  the  nearest  star. 

Make  a  circle,  two  inches  across,  on  the  ground.  Let  this 
circle  represent  our  entire  Solar  System,  with  the  Sun  in  the 
centre.  Then  a  similar  circle  including  the  nearest  star  will, 
on  the  same  scale,  be  1,524  feet  across.  That  is  to  say,  it  will 
make  a  circular  race-track  nearly  a  mile  around. 

If  these  circles  be  regarded  as  solid  spheres  or  globes,  then 
the  larger  one,  representing  the  intense  loneliness  of  our  Solar 
System,  will  be  equal  to  766,000,000,000  globes  like  the  smaller 
one  that  represents  the  size  of  our  system. 

LONG-RANGE  CANNON 

It  is  difficult  for  one  who  is  not  an  astronomer  to  realise  the 
enormous  light-grasping  power  of  our  great  telescopes.  Most  of 
us,  indeed,  fail  to  realise  even  the  power  of  the  unaided  eye  to 
penetrate  space.  Here  are  a  few  illustrations  of  both. 


FIG.  45.  —  YERKES  OBSERVATOKT,   WILLIAMS  BAY,   WISCONSIN 

FIG.  46. —  THE  FORTY-INCH  REFRACTOR  OF  THE  YERKES  OBSERVATORY: 
THE  LARGEST  IN  THE  WORLD 


SOME   MORE   DIMENSIONS  81 

Suppose  that  our  Sun  were  to  leave  us  and  go  off  in  the  direc- 
tion of  Sirius  at  such  a  rate  as  to  double  its  distance  in  one 
year.  And  suppose  it  were  to  continue  receding  at  the  same 
rate  for  an  indefinite  period.  In  100,000  years  it  would  be  about 
as  bright  as  Sirius,  though  the  latter  would  still  be  5|  times  as 
far  away  from  us.  In  550,000  years  it  would  pass  Sirius  and 
appear  to  us  about  as  bright  as  the  Pole  Star.  In  3,000,000 
years  it  would  be  just  visible  to  the  naked  eye,  and  its  light 
would  take  46  years  to  reach  us.  After  receding  for  750,000,000 
years,  it  would  still  be  visible  to  a  telescope  with  a  50-inch  ob- 
ject-glass or  mirror,  and  its  light  would  take  more  than  11,000 
years  to  reach  us. 

Suppose  that  Sirius  were  to  recede  from  us  at  the  rate  of 
1,500,000  miles  per  second  (eight  times  the  speed  of  light),  so  as 
to  double  its  distance  in  one  year.  In  30  years  it  would  be  just 
visible  to  the  naked  eye,  and  its  light  would  take  240  years  to 
reach  us.  After  receding  for  7,500  years  it  would  still  be  visible 
with  a  50-inch  telescope,  and  its  light  would  take  about  60,000 
years  to  reach  us.1 

Suppose  that  a  star  that  is  just  visible  to  the  naked  eye  were 
to  recede  from  us  at  such  an  inconceivable  rate  as  to  double  its 
distance  in  a  year.  It  would  be  about  250  years  before  it  would 
be  lost  sight  of  by  the  same  50-inch  telescope.2 

Let  one  inch  represent  the  distance  of  the  farthest  star  visible 
to  the  naked  eye.  Then  the  distance  at  which  our  largest  tele- 
scopes would  lose  sight  of  it  will  be  represented  by  about  21 
feet. 

This  last  illustration  may  be  put  in  another  form.  If  a  globe 
two  inches  in  diameter  be  supposed  to  contain  all  the  stars 
visible  to  the  naked  eye,  then  it  will  take  a  globe  42  feet  thick 
to  contain  all  the  stars  visible  to  our  great  telescopes. 

1  It  would  be  more  correct  to  say  that  stationary  stars  equal  to  Sirius  would  be 
just  visible  at  those  distances,  and  that  their  light  would  take  so  long  to  reach 
us. 

2  The  same  remark  applies  here. 


82     HOW   TO   KNOW   THE   STARRY   HEAVENS 

ETERNAL  LIGHT 

Suppose  that  (with  the  exception  of  our  Sun)  every  star  in 
the  Universe  were  to  be  blotted  out  of  existence  to-day,  we 
should  not  know  of  it  for  several  years.  We  should  continue  to 
receive  the  light  which  is  already  on  the  way,  and  the  stars 
would  still  appear  to  twinkle  as  they  have  always  done.  In  a 
little  over  four  years  the  nearest  star  would  suddenly  go  out  of 
sight.  But  no  one  would  miss  it  except  the  astronomer.  After 
another  four  years  the  mysterious  disappearance  of  Sirius  would 
attract  more  general  attention.  In  a  century  a  few  more  would 
be  missed,  but  the  majority  would  remain  visible  for  thousands 
of  years.  Some  of  the  stars  seen  in  the  most  powerful  telescopes 
may  be  so  far  off  that  the  light  we  now  see  left  them  when 
Great  Britain  was  part  of  the  mainland  of  Europe,  and  the 
Britons  were  fighting  the  hippopotamus,  the  hyena,  and  the  sabre- 
toothed  tiger. 

REASON  VERSUS  IMAGINATION 

I  have  now  given  sufficient  illustrations  of  the  unthinkable 
vastness  of  the  visible  Universe.  It  is  no  use  trying  to  realise 
even  ascertained  facts  of  such  immensity.  As  Dr.  J.  W.  Draper 
says,  "  distances  and  periods  such  as  these  are  beyond  our  grasp. 
They  prove  to  us  how  far  human  reason  excels  imagination,  the 
one  measuring  and  comparing  things  of  which  the  other  can 
form  no  conception,  but  in  the  attempt  is  utterly  bewildered  and 
lost." 

NUMBERING  THE  STARS 

"  Look  now  toward  heaven, 

And  count  the  stars. 
Tell  the  number  of  them 
If  thou  art  able."  —  Genesis  xv,  5  (A.  Zazel). 

The  next  question  is,  How  many  stars  are  there  in  the  visible 
and  invisible  parts  of  our  Universe  ? 

This,  like  many  other  celestial  problems,  can  be  only  partially 
answered.  The  unaided  eye  can  perceive  about  3,000  in  each 


SOME   MORE   DIMENSIONS  83 

hemisphere.  Argelander's  great  catalogue  contains  a  list  of  6"ver 
300,000,  all  visible  with  a  pocket  telescope.  A  good  three-inch 
achromatic  telescope  brings  about  1,000,000  in  sight.  But  they 
are  shown  by  the  giant  instruments  of  our  great  observatories 
in  such  multitudes  that  they  cannot  be  counted.  We  have  to 
be  satisfied  with  more  or  less  approximate  estimates  of  their 
numbers. 

Yet  every  improvement  in  light-grasping  power  brings  mil- 
lions of  fresh  ones  into  sight,  and  shows  the  "  backspace  "  of  sky 
all  glowing  with  the  light  of  invisible  suns  too  far  off  to  be 
separately  distinguished. 

It  has  of  late  years  been  found  possible  to  attach  a  photo- 
graphic apparatus  to  a  telescope,  so  as  to  make  the  luminous 
bodies  in  the  heavens  take  their  own  pictures.  Whole  groups 
of  stars  are  photographed  on  one  plate.  Complete  sets  of  these 
star-photographs  (embracing  every  nook  and  corner  of  the  celestial 
sphere)  are  taken  every  year,  and  carefully  compared  with  one 
another,  to  find  out  what  changes  are  going  on  in  the  heavens. 
It  will  not  be  long  before  every  star  photographically  visible  to 
the  most  powerful  telescope  will  have  its  present  position  accu- 
rately defined  on  these  photographic  charts. 

By  a  prolonged  exposure  of  the  sensitive  plate  even  invisible 
stars  gradually  photograph  themselves,  so  that  immense  numbers 
of  stars  have  been  discovered  which  no  eye  can  see,  even  with 
the  aid  of  the  same  telescope.  "  Many  ten  thousands  "  of  stars 
have  often  registered  themselves  on  a  single  plate.  In  one 
case  over  400,000  were  actually  counted. 

It  has  been  estimated  that  for  every  star  visible  to  the  naked 
eye  there  are  at  least  50,000  visible  to  the  telescopic  camera. 
This  would  bring  the  number  of  visible  stars  to  about  300,000,000. 

Yet  even  the  picture-taking  power  of  the  photographic  tele- 
scope has  its  limits.  In  photographing  the  Milky  Way  its  plates 
(when  long  exposed)  are  sometimes  clouded  by  constellations 
too  faint,  through  distance,  for  the  individual  stars  to  record 
themselves.  However  much  the  telescope  and  its  adjuncts  may 
be  improved  in  the  future,  they  will  always  fail  to  penetrate 


84      HOW   TO   KNOW   THE   STARRY   HEAVENS 

more  than  a  certain  distance  into  space.     Beyond  that  limit  there 
may  still  be,  as  George  Sterling  suggests,  the  — 

—  "  fires  of  unrecorded  suns 
That  light  a  heaven  not  our  own." 

IS  THE  UNIVERSE  LIMITED   IN  EXTENT? 

"  The  centre  of  space  is  everywhere,  and  its  circumference  nowhere."  —  Blaise 
Pascal. 

We  know,  by  abstract  reasoning,  that  space  is  limitless.  The 
question  arises,  Is  there  a  limit  to  the  distribution  of  luminous 
and  non-luminous  bodies  in  that  limitless  space  ?  In  other 
words,  Is -there  a  limit  to  the  Universe  ? 

Professor  Newcomb  deals  with  the  question  as  follows.  Sup- 
pose a  globe  to  encircle  and  include  all  the  stars  visible  to  the 
naked  eye.  And  suppose  another  of  double  radius,  a  third  of 
treble  radius,  etc.,  with  similar  distribution  of  stars.  As  the 
light  received  from  a  given  luminous  area  or  surface  is  in  the 
inverse  ratio  to  the  square  of  the  distance,  each  shell  will  send 
us  an  equal  amount  of  light.  If  there  is  no  limit,  then,  unless 
some  cause  produces  a  loss  of  light,  the  whole  sky  will  be  as 
bright  as  the  Sun.  As  this  is  very  far  from  being  the  case,  it  is 
evident  that  there  is  either  a  limit  or  a  loss  of  light. 

It  is  not  impossible  that  light  itself  may  have  limits  to  its 
continuous  flight :  that  it  may  be  intercepted  by  dark  bodies  on 
its  way  to  us,  or  that  its  waves  may  lengthen  out  till  all  vibra- 
tion ceases.  Dr.  J.  J.  See  says  on  this  subject : 

11  In  the  exploration  of  the  sidereal  heavens  it  is  found  that  the 
more  power  the  telescope,  the  more  stars  are  disclosed ;  and  hence 
the  practical  indications  are  that  in  most  directions  the  sidereal  system 
extends  on  indefinitely.  But  the  possible  uniform  extinction  of  light 
due  to  the  imperfect  elasticity  of  the  luminiferous  ether,  and  the  un- 
doubted absorption  of  light  by  dark  bodies  widely  diffused  in  space, 
seem  to  preclude  for  ever  a  definite  answer  to  the  question  of  the 
bounds  of  creation." 


FIG.   47.  —  MILKY  WAY   SURROUNDING  MESSIER  II. 
Photographed  with  a  portrait-leiis,  by  Barnard,  at  Yerkes  Observatory. 


FIG.  48.  —  THE  STAR-CLUSTER  MESSIER  II. 
Photographed  with  the  great  Yerkes  refractor,  by  Ritchey. 


SOME   MORE   DIMENSIONS  85 

So  George  N.  Lowe  may  be  right  when  he  says : 

"  Beyond  the  thirty  thousand  years 

Of  light,  the  giant  systems  swing  — 
Vast,  unknown  suns  and  flaming  spheres  — 
And  great  worlds  from  their  girdles  fling." 

INCOMPREHENSIBLE,  YET  TRUE 

I  once  put  the  following  question  to  the  late  Richard  A. 
Proctor : 

"  Let  u» suppose  that  a  man  could  reach,  in  one  second  of  time,  the 
remotest  star  visible.  Let  us  also  suppose  that  he  could  continue  on  at 
the  same*  speed,  in  a  straight  line,  to  all  eternity.  Would  he  ever  get 
to  the  end  of  suns  and  worlds,  or  would  he  always  have  as  many  in 
front  of  him  as  he  had  behind  him  ]  The  latter  proposition  seems 
absurd,  yet  appears  to  be  a  necessary  consequence  of  the  theory  that 
'end  there  is  none,  nor  is  there  yet  beginning.'  But  if,  on  the  other 
hand,  the  last  star  could  be  reached  in  every  direction,  then  the  Uni- 
verse not  only  has  bounds,  but  is  as  a  mere  grain  of  sand  in  the  infinite 
desert  of  space.  Of  course  I  use  the  word  Universe  in  its  largest 
sense." 

Mr.  Proctor's  reply  was  as  follows : 

"  I  fear  that  to  this  question  there  is  but  one  answer,  '  We  don't 
know.'  The  infinite,  which  necessarily  is,  is  necessarily  incompre- 
hensible." l 

QUANTITY   OF   MATTER   IN  UNIVERSE 

Of  late  years  an  attempt  has  been  made  to  ascertain  the  total 
amount  of  matter  in  the  Universe  by  estimating  the  gravita- 
tional force  acting  on  individual  stars.  One  of  these,  known 
as  1,830  Groombridge,  is  moving  at  a  speed  of  200  miles  per 
second.  Let  us  suppose  that  it  has  "  fallen  "  from  a  practically 
infinite  distance,  pulled  by  the  combined  attraction  of  every- 
thing in  our  Universe.  It  has  been  estimated  that  more  than 
30,000,000,000  suns  like  ours  would  be  necessary  to  produce 

1  See  "Knowledge,"  June  1,  1886,  page  254. 


86     HOW   TO   KNOW   THE   STARRY   HEAVENS 

the  observed  speed.  If  our  Sun  be  taken  as  an  average  star, 
this  would  indicate  that,  for  every  star  visible  to  the  telescope, 
there  are  more  than  a  hundred  that  are  invisible,  either  from 
distance  or  because  they  have  ceased  to  be  luminous. 

If  this  method  be  trustworthy,  it  will  give  us  some  idea  as  to 
the  dimensions  of  our  Universe.  But  it  does  not  disprove  the 
existence  of  other  and  similar  universes  at  practically  infinite 
distances  from  ours,  and  from  one  another.  For  if  it  did,  then 
the  observed  motion  of  a  comet  visiting  our  Solar  System  (being 
the  result  of  the  total  attraction  of  the  matter  in  the  system), 
could  be  used  as  a  proof  of  the  non-existence  of  air^hing  out- 
side of  the  Solar  System.  And  this  would  hardly  be  doing 
justice  to  the  myriads  of  Stellar  Systems  which  are  known  to 
surround  our  own. 

The  Law  of  Gravitation,  which  has  just  been  alluded  to,  will 
be  dealt  with  in  Chapter  XV. 

OUTSIDE  UNIVERSES 

The  study  of  the  visible  Universe  shows  that  it  is  composed 
of  ascending  series  of  similar  systems.  For  example  :  (1)  atoms 
appear  to  be  spheroidal  "  star-clusters  "  of  still  smaller  particles 
in  motion ;  (2)  suns  and  worlds  are  rotating  spheroids  built  up 
of  these  atoms ;  (3)  stellar  systems  are  rotating  spheroids  built 
up  of  suns  and  worlds ;  (4)  the  visible  Universe  appears  to  be 
a  rotating  spheroid  built  up  of  a  Milky  Way  of  stellar  systems. 

It  is  possible  that  this  largest  spheroid,  which  we  call  the 
Universe,  may  be  only  one  out  of  innumerable  similar  spheroids, 
rotating  at  practically  infinite  distances  from  each  other,  and 
forming  a  still  vaster  rotating  spheroid. 

These  speculations  could  be  extended  ad  infinitum  at  both 
ends  of  the  series.  It  would,  however,  be  a  waste  of  time  to 
consider  them  seriously,  they  only  serve  to  show  how  little  we 
really  know  of  the  great  "  Kiddle  of  the  Universe." 


CHAPTER  VIII 

THE  PRINCIPLES  AND  APPLICATIONS  OF  THE 
SPECTROSCOPE 

"•  And  Elohim  spake  unto  Noah  and  to  his  sons  with  him,  saying  ...  I  will 
establish  my  covenant  with  you  ;  neither  shall  all  flesh  be  cut  off  any  more  by  the 
waters  of  a  flood  ;  neither  shall  there  any  more  be  a  flood  to  destroy  the  earth. 
And  Elohim  said :  This  is  the  token  of  the  covenant.  .  .  .  I  do  set  my  bow  in  the 
cloud,  and  it  shall  be  for  a  token  of  a  covenant  between  me  and  the  earth,  .  .  . 
and  I  will  look  upon  it,  that  I  may  remember  the  everlasting  covenant."  - 
Genesis  ix,  8-16. 

THE  BRIDGE  OF  BIFROST 

THE  rainbow  has  always  been  a  source  of  wonder  and  ad- 
miration to  mankind.  Before  history  began,  men  were 
blindly  theorising  as  to  the  cause  and  meaning  of  the  wonder- 
ful arch  of  colours  which,  phantom-like,  seemed  to  span  the 
earth  and  reach  to  heaven.  Among  the  old  Norsemen  it  was 
known  as  the  Bridge  of  Bifrost,  and  was  supposed  to  connect 
the  abode  of  the  gods  with  that  of  their  subjects  on  earth.  No 
mortal  could  set  his  foot  on  this  bridge,  but  the  gods  used  it 
when  they  had  business  to  transact  or  mischief  to  work  in  the 
lower  regions. 

In  more  recent  times  its  nature  and  cause  have  been  ascer- 
tained, and  its  glorious  colours  have  been  reproduced  by  allow- 
ing the  Sun  to  shine  through  a  three-cornered  glass  prism. 
Marvellous  to  relate,  the  fragments  of  rainbow  thus  produced 
have  not  only  helped  chemists  to  solve  some  of  the  mysteries 
concerning  matter  on  earth,  but  have  also  enabled  astronomers 
to  wrest  from  high  heaven  many  of  the  secrets  relating  to  the 
chemistry,  constitution,  and  movements  of  the  heavenly  bodies, 
some  of  which  still  bear  the  names  of  the  old  deities. 


88     HOW   TO   KNOW   THE   STARRY   HEAVENS 

If  the  old  Norse  religion  had  only  survived  until  now,  how 
easily  might  the  priests  of  Odin  have  proved  the  inspiration  of 
the  old  records !  They  could  now  show  that  the  apparently 
childish  stories  (though  not  literally  true)  have  a  hidden  sym- 
bolical meaning  and  refer  to  facts  that  only  Gods  could  have 
then  been  acquainted  with. 

But,  alas !  the  new  light  has  come  too  late.  The  old  narra- 
tives have  been  replaced  by  others.  The  Bridge  of  Bifrost  is 
now  the  only  remaining  proof  that  the  life  of  the  world  was 
once  practically  destroyed  by  .an  all-wise  Creator,  in  a  justifiable 
exasperation  at  the  moral  imperfections  with  which  he  had  know- 
ingly endowed  its  most  perfect  inhabitants. 

THE  KEYS  TO  THE  UNIVERSE 

The  rainbow  and  its  artificial  imitations  are  caused  by  the 
refraction  of  light,  a  property  without  which  mankind  would 
have  been  for  ever  left  in  the  dark  as  to  the  outer  Universe. 
The  study  of  the  peculiarities  of  this  refraction  has  given  us  the 
telescope  and  the  spectroscope  —  two  master  keys  which  are 
unlocking  the  mysteries  of  the  Universe  to  man.  The  two  dis- 
coveries have  indeed  been  described  by  enthusiastic  scientists  as 
the  most  important  events  that  have  taken  place  on  earth  since 
history  began.  The  telescope,  the  camera,  the  spectroscope,  and 
even  the  human  eye  itself,  would  have  been  impossible  without 
the  refraction  of  light  upon  which  they  —  and  the  rainbow  — 
depend. 

The  instrument  into  which  the  rainbow-like  spectrum  has 
been  harnessed  is  known  as  the  spectroscope.  A  few  words 
concerning  the  principles  utilised  in  this  instrument  will  help 
us  to  grasp  some  of  the  facts  which  have  been  brought  down 
from  heaven  by  its  means. 

MATTER  AND  ETHER 

There  appear  to  be  in  Nature  two  forms  of  substance  entirely 
distinct  from  each  other  in  structure  and  functions.  Every 


THE   SPECTROSCOPE  89 

nook  and  corner  of  infinite  space  appears  to  be  packed  with  one 
or  the  other  of  these  two  forms  of  substance. 

I.  Matter.  —  That  with  which  we  are  most  familiar  is  com- 
monly known  as  matter.     It  is  made  up  of  atoms,  possesses 
weight,  and  exists  as  a  solid,  liquid,  or  gas,  according  to  sur- 
rounding conditions. 

II.  Ether.  —  The  other  goes  by  the  name  of  luminiferous  (or 
light-bearing)  ether.     This  ether  is  not  made  up  of  atoms,  but 
appears  to  consist  of  homogeneous  particles  or  corpuscles.    These 
are  estimated  to  be  about  a  thousand  times  less  (in  mass)  than 
the  smallest  atoms  of  ordinary  matter.     On  account  of   their 
being  the   carriers   of  so-called   negative   electricity  they  are 
sometimes  called  negative  particles. 

This  ether  is  practically  without  weight,  being  millions  of  mil- 
lions of  times  thinner  than  air.  It  probably  fills  to  saturation 
all  space  which  is  not  occupied  by  ordinary  matter^  even  filling 
the  little  spaces  between  the  atoms  of  matter.  Some  of  the 
phenomena  produced  by  it  are  explainable  only  on  the  theory 
that  it  can  pass  through  suns  and  worlds  as  water  passes 
through  a  sieve,  or  as  light  passes  through  a  massive  sheet 
of  glass.1 

VIBRATIONS  OF  ETHER 

All  matter  has  a  life  of  its  own  and  is  in  continuous  motion. 
The  various  motions  of  matter  produce  corresponding  vibrations 
in  the  ether,  which  carries  them  to  inconceivable  distances. 
Some  of  these  vibrations  become  sensible  to  us  in  the  form  of 
light;  others  are  recognisable  as  heat,  electricity,  magnetism, 
etc. 

REFRACTION  OF  LIGHT 

Light  may  be  regarded  as  produced  by  wave-like  undulations 
of  the  elusive  ether  which  appears  to  fill  all  space.  The  un- 

1  This  may  at  first  seem  incredible,  even  with  our  recent  experience  of  Roentgen 
rays.  But  it  must  be  remembered  that  it  is  not  a  whit  more  wonderful  than  the 
commonplace  yet  astounding  fact  that  light  can  pass,  almost  without  let  or  hin- 
drance, through  solid  glass  or  ice. 


90      HOW   TO   KNOW   THE   STARRY   HEAVENS 

dulations  of  light  radiate  through  this  ether  like  the  ripples 
which  spread  over  water  when  a  pebble  is  thrown  into  it.  Or 
they  may  more  correctly  be  likened  unto  the  vibrations  of  air 
which  start  out  in  all  directions  from  a  centre  of  vibration  and 
ultimately  become  sensible  to  the  ear  as  sound.  Like  the  waves 
of  sound,  the  ethereal  undulations  of  light  vary  in  length.  That 
is  to  say,  the  interval  between  the  summit  of  one  wave  and  that 
of  the  next  varies  in  length  in  different  kinds  of  light.  When 


FIG.  49.  — A  PRISM  AND  ITS  SPECTRUM 

a  ray  of  white  light  passes  through  a  glass  prism,  the  greater 
density  of  the  medium  causes  it  to  be  bent  or  refracted.  The 
light,  instead  of  coming  out  opposite  to  where  it  entered  the 
prism,  comes  out  at  an  angle,  the  bend  being  toward  the  side 
where  the  prism  is  thickest.  The  inequality  of  wave-lengths 
causes  this  refraction  to  be  unequal  also,  and  the  white  light 
splits  up  into  an  infinite  number  of  shades,  the  best  known  of 
which  go  by  the  names  of  violet,  indigo,  blue,  green,  yellow, 
orange,  and  red  (see  Figure  49). 

Of  these  colours  the  violet  rays  have  the  shortest  wave-length 
and  are  bent  the  most  from  their  previous  course.  The  red 
waves  are  the  longest,  and  are  bent  the  least. 

This  visible  spectrum  is  only  a  part  of  a  much  longer  spectrum, 
the  rest  of  which  is  invisible  to  the  eye,  though  recognisable  by 
other  means.  The  short  waves  at  the  violet  end  are  character- 
ised by  the  intensity  of  their  actinic  or  chemical  properties, 


THE   SPECTROSCOPE  91 

while  the  long  ones  at  the  red  end  give  out  more  heat.  The 
violet  waves  are  so  short  that  61,000  of  them  reach  only  an 
inch,  while  the  red  ones  are  so  long  that  there  are  only  33,000 
of  them  to  an  inch.  They  all  travel  at  the  same  speed,  186,000 
miles  in  a  second  of  time.1 

THE   SPECTROSCOPE 

For  convenience  the  glass  prism  is  placed  at  the  elbow  of  an 
instrument  that  looks  like  a  bent  telescope  and  is  commonly 


FIG.  50.  —  A  OXE-PKISM  SPECTROSCOPE 

known  as  a  spectroscope  (see  Figures  50,  51,  and  52).  A  ray  of 
white  light  entering  at  one  end  through  a  narrow  slit  passes 
along  the  tube  until  it  comes  to  the  prism.  There  it  is  bent 
and  split  up  into  the  colours  of  the  rainbow.  This  row  of 
colours  is  known  as  a  spectrum,  and  is  examined  directly  from 
the  other  end  of  the  "  bent  telescope." 

The  spectroscopes  now  used  are  more  complicated  than  the 
above,  with  a  "  battery  "  of  prisms  (or  with  a  corrugated  mirror 

1  It  is  interesting  to  note  that  if  the  prism,  instead  of  being  straight,  is  bent 
around  a  centre  into  a  lens-form,  it 'brings  the  light  to  a  focus,  and  forms  the 
basis  of  the  telescope  and  microscope.  In  these  instruments  special  means  are 
taken  to  avoid  the  separation  of  the  various  colours,  but  in  the  spectroscope  spe- 
cial means  are  taken  to  increase  the  dispersion. 


92      HOW   TO   KNOW   THE   STARRY   HEAVENS 

called  a  diffraction  grating),  but  they  all  depend  on  the  same 
principle,  so  need  not  be  here  described. 

VARIETIES  OF  SPECTRA 

Now  it  is  found  that  different  sources  of  light  do  not  give 
the  same  spectrum  when  examined  through  the  spectroscope. 
There  are,  in  fact,  several  distinct  classes  of  spectra,  which  are 
here  given. 

I.  Continuous  Spectrum.  —  Light  from  an  incandescent  sub- 
stance in  a  solid  or  liquid  state,  or  from  a  glowing  gas  which  is 
under  great  pressure,  gives  a  continuous  spectrum,  all  the  colours 


FIG.  51.  —  SECTION  OF  A  ONE-PRISM  SPECTROSCOPE 

being  fully  and  evenly  represented.     In  this  case  it  is  evident 
that  light  of  every  wave-length  is  being  given  off  (see  Figure  5  3,  a). 

II.  Emission  or  Radiation  Spectrum.  —  Light  from  an  incan- 
descent gas  which  is  uncompressed,  and  therefore  free  to  vibrate 
at  its  own  rate,  does  not  give  a  continuous  spectrum,  as  in  the 
first  instance.     It  merely  gives  a  limited  number  of  bright  lines, 
crossing  the  long  streak  where  the  spectrum  should  have  been. 
These  bright  lines  vary  in  colour  according  to  their  position  in 
the  (absent)  spectrum.     This  shows  that  a  glowing  gas,  when 
unconfined,  only  gives  out  light  of  certain  definite  wave-lengths 
(see  Figure  53,  b,  d,  e). 

III.  Absorption  Spectrum.  —  When  light  of   the  first  class 
passes  through  a  mass  of  uncompressed  gas,  its  otherwise  con- 
tinuous spectrum  becomes  to  a  certain  extent  discontinuous. 
It  is  crossed  by  certain  dark  lines,  making  the  spectrum  look  as 
though  there  were  a  ladder  in  front  of  it,  with  numerous  black 
rungs  at  irregular  distances  from  one  another.     This  shows  that 
light  of  certain  wave-lengths  is  absent,  being  absorbed  by  the 


THE   SPECTROSCOPE  93 

uncompressed  gas  through  which  the  light  passes  (see  Figure 
53,  i,  etc.). 

IV.  When  the  source  of  light  is  partly  solid  or  compressed 
matter,  and  partly  uncompressed  gas,  the  result  is  a  spectrum 
crossed  by  lot h  bright  and  dark  lines  or  bands  (see  Figure  53,  h). 


FIG.  52.  —  A  COMPOUND  SPECTROSCOPE 

SPECTRUM  ANALYSIS 

Chemists  have  discovered  that  there  are  only  about  eighty 
different  kinds  of  matter  on  the  accessible  parts  of  our  globe, 
and  that  only  about  a  quarter  of  these  are  abundant  or  im- 
portant. All  the  tens  of  thousands  of  different  substances  on 
earth  (whether  solid,  liquid,  or  gaseous)  consist  of  these  elements, 
either  separate  or  in  partnership  with  one  another. 

Now  the  great  importance  of  the  spectroscope  depends  on 
the  fact  that  every  one  of  the  elements,  when  in  the  condition 


94     HOW  TO   KNOW   THE   STARRY   HEAVENS 

of  glowing  gas,  produces  its  own  special  lines  in  the  spectro- 
scope. By  examining  the  spectrum  of  an  unknown  substance, 
the  elements  of  which  it  is  composed  can  be  ascertained.  This 
way  of  examining  substances  is  therefore  known  as  spectrum 
analysis. 

It  has  also  been  found  that  when  light  passes  through  a  cer- 
tain uncompressed  gas,  that  gas  absorbs  the  same  rays  of  light 
which  it  naturally  emits  when  it  is  itself  an  uncompressed 
source  of  light  (see  Figure  53,  b,  c). 

Bearing  these  facts  in  mind,  let  us  turn  back  to  reconsider 
the  four  classes  of  spectra  given  above. 

When  the  spectrum  is  of  the  first  class  (continuous),  we  do 
not  get  much  information  with  regard  to  the  source  of  light. 
We  know  that  the  light  does  not  proceed  from  an  uncompressed 
gas,  but  we  cannot  tell  whether  it  is  given  off  by  a  solid,  a 
liquid,  or  by  a  compressed  gas. 

When  it  is  of  the  second  class  (emission),  consisting  entirely 
of  a  limited  number  of  bright  lines,  we  know  that  the  light  pro- 
ceeds from  a  glowing  gas  that  is  not  under  great  pressure.  And 
we  can  tell  what  particular  elements  that  gas  consists  of  or  con- 
tarns.  Certain  lines  prove  that  it  contains  hydrogen;  other 
lines  tell  of  the  presence  of  sodium.  Another  set  is  caused  by 
iron  vapour;  and  so  on,  with  all  the  elements  (see  Figure  53, 
b,d,e,f). 

When  the  spectrum  is  of  the  third  class  (absorption),  it  is 
nearly  continuous,  but  is  crossed  by  dark  lines.  In  this  case 
we  know  that  the  luminous  body,  whatever  it  is,  is  surrounded 
by  uncompressed  gas  which  has  absorbed  certain  light-waves 
and  so  prevented  them  from  reaching  us,  and,  as  in  the  second 
class,  we  can  tell  what  particular  elements  that  gas  consists  of 
or  contains ;  for  the  element  which  produced  a  certain  set  of 
bright  lines  when  it  was  a  source  of  light  produces  the  same  set 
of  dark  lines  when  it  surrounds  the  source  of  light  or  comes 
between  the  source  of  light  and  its  spectrum.  Hydrogen, 
sodium,  iron,  etc.,  are  in  the  one  case  recognised  by  their  pecu- 
liar bright  lines,  and  in  the  other  case  by  identical  sets  of 


THE   SPECTROSCOPE  95 

dark  lines.  The  difference  is  one  of  condition,  not  of  substance 
(see  Figure  53,  6,  c). 

A  spectrum  of  the  fourth  class,  crossed  by  both  bright  and 
dark  lines  or  bands,  denotes  that  the  source  of  light  is  either  a 
mixture  of  solid  and  gas,  or  of  compressed  and  uncompressed 
gases.  The  different  sets  of  lines  denote  the  presence  and  con- 
dition of  their  respective  elements,  as  in  the  other  classes. 

In  spite  of  the  one  element  of  uncertainty  mentioned,  the 
above  results  are  remarkable  achievements,  even  when  applied 
to  substances  actually  present  in  the  chemist's  laboratory. 
They  have  led  to  the  discovery  of  hitherto  unknown  elements 
and  to  a  great  advance  in  chemistry  generally.  But  still  more 
remarkable  is  the  fact  that  this  spectrum  analysis  can  be  applied 
to  a  distant  source  of  light.  Large  numbers  of  the  terrestrial 
elements  have  been  recognised  in  the  sierra  or  atmosphere  of 
our  Sun,  and  even  the  stars  are  not  too  far  off  to  be  cross- 
examined  by  the  spectroscope  as  to  their  chemical  composition 
and  physical  condition. 

The  spectrum  of  a  star  being  very  faint,  it  is  observed  by  a 
spectroscope  attached  to  a  telescope,  the  combination  being 
known  as  a  tele-spectroscope  (see  Figure  54).  Sometimes  the 
spectrum  is  photographed  for  future  .examination  and  com- 
parison. The  instrument  which  accomplishes  this  is  termed  a 
spectrograph  (see  Figure  55). 

SPECTRA  OF  STARS 

Our  Sun  and  the  brighter  stars,  when  examined  with  the  spec- 
troscope, give  spectra  of  the  third  class  (absorption).  That  is  to 
say,  their  spectra  are  fairly  continuous,  but  are  crossed  by  dark 
lines.  This  shows  that  such  stars  are  all  related  to  one  another, 
so  that  the  study  of  any  one  of  them  will  throw  light  on  all  the 
rest.  They  are  all  suns,  somewhat  similar  to,  but  not  identical 
with,  our  Sun  (see  Figure  53,  i,j,  Jc,  I,  m). 

The  fainter  stars  have  not  yet  been  satisfactorily  examined 
with  the  tele-spectroscope,  on  account  of  the  small  quantity  of 
light  we  receive  from  them.  Still,  a  certain  amount  of  infor- 


96     HOW  TO   KNOW  THE   STARRY   HEAVENS 

mation  has  been  obtained  concerning  their   composition  and 
condition.  - 

SPECTRA  OF  NEBULAE 

When  the  heavens  are  examined  with  the  help  of  a  telescope 
of  some  considerable  power,  enormous  numbers  of  stars  are 
revealed.  But,  besides  these  stars,  the  telescope  also  brings 
into  sight  large  numbers  of  tiny  patches  of  hazy  light.  These 
look  as  though  they  might  be  clusters  of  stars  which  are  too 
far  off  to  be  separately  distinguished.  From  their  cloud-like 
appearance  these  streaks  and  patches  are  termed  nebulce.1 

With  two  or  three  exceptions  these  nebulae  are  not  visible  to 
the  naked  eye,  and  the  vast  majority  of  them  require  very 
powerful  instruments  to  bring  them  into  view. 

Modern  telescopes  have  in  many  cases  proved  that  they  are 
really  composed  of  stars,  and  every  increase  in  telescopic  power 
has  resolved  fresh  nebulse  into  star-clusters.  At  one  time  it 
was  thought  that  all  would  eventually  be  proved  to  be  nothing 
more  or  less  than  clusters  of  faint  stars.  It  has  been  ascer- 
tained, however,  that  there  are  two  main  classes  of  so-called 
nebulse,  and  that  only  those  of  one  class  are  composed  of  stars. 

PSEUDO-NEBULAE  (Star- Clusters).  —  All  of  those  nebulse  which 
have  been  shown  by  the  telescope  to  be  really  star-clusters  give 
absorption  spectra  similar  to  those  of  the  visible  stars  (Class  III), 
and  some  nebulae  which  have  not  yet  been  resolved  into  stars  by 
the  telescope  have  been  found  to  give  similar  spectra.  As  a 
rule  the  spectrum  of  a  star-cluster  is  so  faint  that  it  consists  of 
a  mere  streak  of  light  in  which  the  colours  and  lines  are  imper- 
ceptible. 

TRUE  OR  GASEOUS  NEBULAE.  —  Other  nebulas,  however,  show 
by  their  bright-line  spectra  (Class  II)  that  they  are  not  com- 
posed of  suns,  but  are  immense  masses  of  glowing  gas,  not  under 
pressure.  Those  which  are  comparatively  bright  give  a  number 
of  bright  lines,  five  of  which  are  much  more  prominent  than  the 

1  The  Latin  word  nebula  means  a  cloud,  fog,  mist,  or  smoke.  It  may  be  regarded 
as  a  diminutive  of  nubes,a.  cloud. 


FIG.   54.  —  TELE-SPECTROSCOPE 
Attached  to  a  telescope  for  the  purpose  of  viewing  celestial  spectra.     Lowe  Observatory. 


FIG.  55.  —  THE  MILLS  SPECTROGKAPH  AT  LICK  OBSERVATORY 
For  photographing  spectra  of  celestial  objects. 


THE   SPECTROSCOPE  97 

rest.  In  the  fainter  nebulae  only  one  of  the  lines  is  visible, 
owing  to  the  small  quantity  of  light  which  reaches  us ;  but  it  is 
readily  distinguished  from  the  practically  continuous  spectrum 
given  by  star-clusters,  etc. 


CLASSES  OF    STARS,   ETC. 

Stars  and  other  self-luminous  heavenly  bodies  have  been 
divided  into  a  number  of  classes,  according  to  their  colours  and 
spectra ;  but  no  hard-and-fast  line  can  be  drawn  between  them, 
as  there  are  many  which  give  intermediate  spectra.  The  fol- 
lowing are  perhaps  the  best  defined  classes,  starting  with  the 
youngest. 

A.  BLUE-GREEN  NEBULA  (G-aseous  Nebulce).  —  Nearly  a  hun- 
dred of  the  nebulae  have  a  bluish-green  tint  which  distinguishes 
them  from  all  others.  They  give  an  emission  spectrum,  con- 
sisting entirely  of  a  few  bright  lines  (Class  II)  on  a  dark  back- 
ground. This  indicates  that  they  are  entirely  composed  of 
uncondensed  gases,  so  extremely  diffuse  as  to  be  transparent  in 
spite  of  their  thickness,  which  is  sometimes  many  millions  of 
miles.  The  lines  are  those  belonging  to  hydrogen,  helium,  and 
"  nebulum  "  (see  Figure  53,  g).  The  lines  are  thin,  denoting  a 
comparatively  low  temperature  and  the  absence  of  pressure. 
One  of  these  lines,  in  the  blue-green  part  of  the  spectrum,  is  so 
bright  that  it  gives  the  blue-green  tinge  peculiar  to  nebulas  of 
this  class  when  seen  through  the  telescope.  It  is  due  to  an 
unknown  substance  which  has  been  named  "  nebulum." 

About  half  of  these  gaseous  objects  are  termed  planetary 
nebulce,  from  the  symmetrical  planet-like  disc  which  they  ex- 
hibit when  seen  through  a  powerful  telescope  (see  Figure  99).1 
The  rest  are  generally  irregular  and  shapeless,  like  the  Great 
Nebula  in  Orion  (see  Figure  68),  and  the  Trifid  Nebula  in  Sagit- 
tarius (see  Figure  67).  The  so-called  King  Nebula  in  Lyra  is 
also  gaseous.  Recent  photographs  taken  at  the  Ann  Arbor 
Observatory  by  Professor  Schaeberli  show  that  its  resemblance 

1  Some  of  the  planetary  nebula  have  a  star-like  nucleus  in  the  centre, 

7 


98      HOW   TO   KNOW   THE   STARRY   HEAVENS 

to  a  ring  is  only  apparent,  and  that  it  is  really  a  spiral  nebula 
(see  Figure  66). 

B.  PEAKLY- WHITE  NEBULAE.  —  Some  thousands  of  nebulae 
have  a  pearly-white  appearance  when  seen  through  the  tele- 
scope.    They  give  a  continuous  spectrum,  which  denotes  that 
they  are  either  composed  of  matter  in  a  solid  state,  or  of  gases 
under  high  pressure  or  great  heat.     Very  little  is  at  present 
known   as  to  the  actual  condition  of  the  matter   composing 
these  nebula3.     But  in  many  cases  it  appears  to  be  collecting 
in  "  centres  of  condensation,"  as  though  stars  were  in  process 
of  formation  out  of  nebulous  matter. 

The  Great  Nebula  in  Andromeda  (see  Figure  64)  belongs  to 
this  class,  as  do  the  spiral  nebulce  generally  (see  Figure  63). 
The  late  Dr.  Keeler  estimated  that  there  are  at  least  120,000 
nebulae  within  range  of  the  telescopic  camera,  and  that  more 
than  half  of  these  show  signs  of  a  spiral  structure. 

C.  NEBULOUS  STARS  (Orion  Stars).  —  These  consist  of  a  faint 
nebulous  haze   with  a  star  in  the  centre.     They  give  broader 
hydrogen  and  helium  lines  than  Class  A,  showing  that  the  gases 
are  hotter  and  more  condensed.     These  stars  are  still  in  process 
of  formation,  and  have  not  yet  drawn  in  all  of  the  nebulous 
matter  around  them.    They  are  therefore  buried  in  the  depths  of 
a  luminous  haze  consisting  chiefly  of  hydrogen   and  helium. 
Examples,  the  stars  in  the  Pleiades  and  those  connected  with 
the  Great  Nebula  in  Orion. 

D.  BLUISH-WHITE  STARS  (Sirian  Stars).  —  About  75   per 
cent  of  the  brighter  stars  appear  to  belong  to  this  class.     Their 
spectra  show  all  seven  colours  (Class  III),  but  the  violet  end  is 
the  most  brilliant.     There  are  four  broad  dark  lines  belonging 
to  hydrogen,  greatly  condensed  and  very  hot.     The  metal  lines 
are  few  and  faint.     One  of  the  magnesium  lines  indicates  a 
temperature   about   equal  to  that  of  the  electric  spark,  —  say 
about  20,000°  F.     The  probability  is  that  these  stars  are  still 
surrounded  by  a  very  deep,  dense,  and  hot  atmosphere  (chromo- 
sphere) of  hydrogen,  and  that  the  high  temperature  prevents 
the  formation  in  it  of  a  bright  cloud-photosphere  of  calcium,  etc. 


w    8 


THE   SPECTROSCOPE  99 

Examples,  Sirius,  Vega,  Altair,  Rigel,  and  Arided  (see  Figure 

53,y,*> 

E.  YELLOWISH  STARS  (Solar  Stars).  —  This  class  includes 
about  23  per  cent  of  the  brighter  stars.  Their  spectra  show  all 
seven  colours  (Class  III),  the  middle  being  the  most  brilliant. 
The  hydrogen  lines  are  faint,  denoting  a  shallower  atmosphere 
or  chromosphere.  There  are  large  numbers  of  very  strong  dark 
absorption  lines,  due  to  the  presence  of  different  metals  (in  the 
gaseous  state)  in  the  stellar  atmospheres.  Many  of  these  metal- 
lic elements  have  been  recognised  by  their  special  lines.  One 
of  the  magnesium  lines  indicates  a  temperature  not  very  far 
from  that  of  the  electric  arc  (6,300°  F.),  or  at  least  it  indicates 
a  temperature  lower  than  that  of  the  electric  spark.  Probably 
12,000°  F.  is  not  very  far  from  the  mark.  The  comparatively 
low  temperature  of  the  chromosphere  —  due  to  its  shallowness 
—  causes  the  formation  of  a  layer  of  bright  clouds  known  as 
the  photosphere.  This  is  composed  of  incandescent  solid  (or 
liquid)  particles  of  calcium,  etc. 

If  all  the  light  from  this  brilliant  photosphere  reached  us, 
the  Sun  and  solar  stars  would  all  be  of  a  pure  white,  and  their 
spectra  would  be  continuous  (Class  I),  containing  rays  of  all 
wave-lengths.  But  as  they  are  surrounded  by  a  chromosphere 
(or  atmosphere)  of  metallic  gases  at  a  rather  lower  temperature, 
some  of  the  light  is  absorbed.  The  absorption  is  greatest  among 
the  short  waves,  so  that  these  stars  all  have  a  yellowish  tinge. 
Their  spectra  are  therefore  of  Class  III,  with  many  dark  absorp- 
tion lines  due  to  the  metallic  gases  in  their  chromospheres. 
Examples,  Capella,  our  Sun,  Procyon,  Pollux,  and  Arcturus 
(see  Figure  53, 1 ;  also  Figure  56). 

F.  ORANGE-RED  STARS.  —  This  class  includes  about  one  per 
cent  of  the  brighter  stars.  More  than  60  of  them  are  irregular 
variables  of  the  Mira  Ceti  class.  The  spectra  show  all  the  seven 
colours  (Class  III),  with  the  red  end  most  brilliant.  There  are 
no  hydrogen  lines  except  when  the  stars  are  at  their  maximum 
of  brilliancy.  The  metal  lines  are  complex,  and  there  are  dark 
absorption-bands,  with  a  sharp,  well-defined  edge  toward  the 


100     HOW  TO   KNOW  THE   STARRY   HEAVENS 

violet.  These  indicate  a  dense  and  complex  atmosphere  of  rela- 
tively low  temperature,  with  chemical  compounds.  Examples, 
Aldebaran,  Betelgeuse,  Antares,  and  Mira  Ceti  (see  Figure 
53,  i). 

G.  BLOOD-RED  STARS.  —  The  spectra  are  similar  to  those  of 
Class  F,  but  are  more  or  less  cut  up  by  dark  absorption-bands. 
These  have  a  sharp,  well-defined  edge  toward  the  red  end  of  the 
spectrum,  and  are  due  to  hydrocarbon  vapours  in  the  stellar 
atmospheres.  In  some  stars  these  bands  are  so  broad  and  dark 
that  but  little  of  the  spectrum  can  be  seen.  Their  light  is  in 
fact  dying  out.  Examples,  Mu  Ceph,  and  152  Schjellerup  (see 
Figure  53,  m ;  also  Figure  57). 

H.  DARK  SUNS.  —  These  give  out  little  or  no  light  and  heat, 
and  therefore  have  no  visible  spectra.  They  are  dead  or  dying 
suns.  We  know  of  their  existence  only  when  they  have  visible 
companions  moving  around  the  same  centre  of  gravity.  Ex- 
ample, one  of  the  companions  of  the  Pole  Star. 

RADIAL  MOTIONS 

Of  late  years  the  tele-spectroscope  has  given  aid  to  astronomy 
in  another  and  unexpected  direction.  Astronomers  have  long 
been  able  to  watch  and  measure  the  movements  (both  real  and 
apparent)  of  the  heavenly  bodies  when  those  movements  were 
more  or  less  at  right  angles  to  the  line  of  sight.  But  until 
recently  they  were  unable  to  detect  motions  toward  or  away 
from  us,  except  indirectly,  by  the  increase  or  decrease  of  bril- 
liancy and  size. 

A  few  years  ago,  however,  it  was  discovered  that,  when  a 
source  of  light  is  approaching,  the  lines  which  cross  its  spectrum 
are  displaced  toward  the  violet  end,  and  that  when  it  is  receding 
they  are  displaced  toward  the  red  end  of  the  spectrum. 

This  is  due  to  the  same  cause  that  makes  a  locomotive  whistle 
appear  to  fall  in  pitch  the  moment  it  passes  the  listener.  While 
the  engine  was  approaching  the  listener,  the  waves  of  sound 
reached  his  ear  at  shorter  intervals  than  they  would  otherwise 


fa    -U 


THE   SPECTROSCOPE  101 

have  done,  and  after  it  has  passed  him  the  reverse  is  the  case. 
Hence  the  apparent  change  of  pitch. 

In  the  case  of  light  (spectroscopically  observed),  j  the  effect  is 
a  slight  change  of  position  instead  of  pitch:,  t<yh$;wi\v$s  aieiu 
fact  crowded  together  and  made  shorter  from,  crest ,  to,  jerest. . 
This  could  not  be  measured  or  even  recognised  in -a  cont^u-yds 
spectrum,  but  in  a  banded  one  it  is  done  with  comparative  ease 
by  so  arranging  as  to  have  a  normal  spectrum  on  each  side  of  it 
for  comparison  (see  Figure  58).  The  result  is  that  we  can  not 
only  tell  that  a  certain  object  is  approaching  or  receding  from 
us,  but  can  also  tell  how  many  miles  it  is  moving  in  those 
directions  per  second. 

This  method  of  detecting  and  measuring  radial  motions  (real 
or  apparent)  can  be  applied  not  only  to  those  heavenly  bodies 
whose  distances  are  known,  but  also  to  those  which  are  utterly 
beyond  the  reach  of  our  space-measuring  instruments.  The 
only  limit  is  that  caused  by  deficiency  of  light. 

It  is  possible  that  this  principle  may  in  time  be  found  avail- 
able for  measuring  star-distances  which  are  too  great  for  our 
present  methods.  It  is  proposed  to  measure  the  orbits  of  double 
stars  spectroscopically,  and  then  use  the  major  axes  as  base 
lines. 

At  present,  stellar  distances  can  be  roughly  estimated  up  to 
60  light-years,  which  is  15  times  the  distance  of  Alpha  Cen- 
tauri.  Dr.  See  thinks  that  some  day  star-distances  of  1,000 
light-years  may  be  determined  by  this  plan. 


CHAPTER  IX 

A  STAR-SPANGLED  BANNER 

"  The  winter  sunset  fronts  the  North, 
The  light  deserts  the  quiet  sky, 
From  their  far  gates  how  silently 
The  stars  of  evening  tremble  forth  ! 

"  Time,  to  thy  sight  what  peace  they  share 
On  Night's  inviolable  breast ! 
Remote  in  solitudes  of  rest, 
Afar  from  human  change  or  care. 

"Eternity,  unto  thine  eyes 

In  war's  unrest  their  legions  surge, 
Foam  of  the  cosmic  tides  that  urge 
The  battle  of  contending  skies."" 

—  George  Sterling,  "  The  Testimony  of  the  Suns." 

STAR  MAGNITUDES 

"  One  star  differeth  from  another  star  in  glory."  —  I  Cor.  xv,  41. 

THE  stars  are  at  such  vast  distances  from  us  that  in  spite 
of  the  enormous  sizes  of  some  of  them  they  all  appear  as 
mere  points  of  light.  This  is  true  even  with  the  most  power- 
ful telescope  in  existence.  Only  one  star  in  the  heavens  is 
near  enough  to  show  a  measurable  disc,  and  that  is  the  one 
which  rules  our  own  system  under  the  names  of  Helios, 
Shemish,  Baldur,  Father  Sol,  the  Sun  of  Kighteousness,  and  a 
thousand  other  suggestive  names. 

But  this  does  not  mean  that  the  stars  all  look  alike,  either 
with  the  naked  eye  or  with  the  telescope.  There  are  stars  of 
almost  all  colours,  and  even  those  which  appear  to  be  of  the 
same  colour  differ  enormously  in  brilliancy.  Sirius  scintillates 
in  the  sky  like  a  sparkling  jewel,  and  glares  down  the  telescope 


A  STAR-SPANGLED  BANNER  103 

tube  like  a  little  sun,  so  that  he  is  better  examined  with  a  dark 
glass.  A  few  other  stars  are  not  far  behind  in  brilliancy. 
Large  numbers  are  fairly  bright,  while  swarms  of  them  are  com- 
paratively inconspicuous.  Still  larger  numbers  are  just  visible 
to  the  naked  eye,  and  the  telescope  reveals  them  in  multitudes 
that  no  man  can  number.  Yet  no  two  of  them  send  us  exactly 
the  same  amount  or  quality  of  light. 

It  has  been  found  convenient  to  classify  the  stars  according 
to  their  brilliancy.  At  first  the  classification  was  rude,  but  of 
late  years  they  have  been  sorted  out  with  considerable  accuracy. 
They  are  divided  into  a  number  of  magnitudes,  a  star  of  one 
magnitude  being  2  J  times  as  bright  as  one  of  the  magnitude 
below  it. l 

Thus  a  star  of  the  first  magnitude  sends  us  100  times  as 
much  light  as  one  of  the  sixth  magnitude,  which  is  the  faintest 
star  visible  without  a  telescope.  And  that  is  100  times  as  bright 
as  one  of  the  eleventh  magnitude,  which  in  its  turn  is  100  times 
as  bright  as  one  of  the  sixteenth.  This  rule  applies  to  all  magni- 
tudes. Taking  Aldebaran  as  a  standard  star  of  the  first  mag- 
nitude, the  North  Pole  Star  belongs  to  the  second,  and  the 
brightest  star  of  the  Pleiades  to  the  third.  The  rest  of  the 
bright  stars  in  the  same  group  belong  to  the  fourth.  The 
faintest  stars  usually  seen  belong  to  the  fifth,  and  those  just 
visible  by  direct  vision  on  very  clear  nights  are  of  the  sixth 
magnitude.  The  seventh  magnitude  stars  can  just  be  glimpsed 
by  oblique  vision,  by  unusually  keen  eyes,  on  exceptionally 
clear  occasions. 

There  are  a  few  stars  brighter  than  Aldebaran,  and  these  are 
sorted  out  in  the  same  way,  with  the  numbers  reversed  and  the 
minus  sign  (— )  placed  in  front  of  them,  instead  of  the  plus  sign 
(+),  which  is  sometimes  used  to  denote  the  regular  magnitudes. 
Thus  one  magnitude  brighter  than  the  first  is  denoted  as  (0), 
the  next  (—1),  and  so  on.  On  this  plan  Sirius  is  denoted  by 
the  sign  (—1.4),  his  brilliancy  being  9.1  times  greater  than  that 
of  Aldebaran.  And  our  Sun  is  a  star  of  the  magnitude  —26.4, 

!More  exactly,  2.512. 


104     HOW   TO   KNOW   THE   STARRY   HEAVENS 

his  apparent  brilliancy  being  about  90,000,000,000  times  greater 
than  that  of  Aldebaran. 

It  must  be  understood  that  the  magnitude  of  a  star  does  not 
denote  its  real  size,  or  its  weight,  or  the  actual  amount  of  light 
given  off  by  it,  or  its  distance  from  us.  It  merely  denotes  the 
amount  of  light  we  receive  from  it,  and  this  depends  on  a  com- 
bination of  these  factors,  which  are  unknown  to  us  in  the  vast 
majority  of  cases.  For  example,  Canopus,  which  is  the  second 
brightest  star  in  the  heavens,  is  at  an  immeasurable  distance 
from  us,  and  must  therefore  be  many  thousands  of  times  larger 
and  brighter  than  our  Sun.  And  some  of  the  fainter  stars 
visible  to  the  telescope  may  be  even  larger  than  Canopus.  On 
the  other  hand,  some  fairly  brilliant  stars  are  smaller  even  than 
our  Sun,  their  distances  from  us  being  comparatively  small,  as 
stellar  distances  go. 

For  this  reason  the  actual  and  apparent  motions  of  the  stars 
form  a  better  guide  to  their  distance,  size,  and  actual  brilliancy 
than  do  their  magnitudes.  We  have  an  example  of  this  in  the 
star  known  as  61  Cygni,  which  is  nearer  to  us  than  Sirius, 
although  it  is  nearly  seven  magnitudes  smaller  and  sends  us 
nearly  600  times  less  light. 

The  number  of  visible  stars  has  been  already  dealt  with.  It 
may,  however,  be  interesting  to  know  that  there  are  about  50 
stars  of  the  second  magnitude  (from  1.6  to  2.4),  and  about  one 
third  that  number  of  larger  ones.  Also  that,  from  this  down, 
each  magnitude  has  about  three  times  as  many  as  the  one 
above  it.  To  the  fourteenth  magnitude  (14.5)  this  would  give 
200,000,000,  stars.  With  the  fainter  stars,  however,  the  in- 
crease in  numbers  appears  to  be  less  rapid.  The  faintest  stars 
visible  to  the  eye  in  the  great  Lick  telescope  are  of  about  the 
seventeenth  magnitude. 

ACTUAL  BRILLIANCY  OF  VISIBLE  STARS 

The  actual  magnitudes  of  the  stars  may  be  considered  from 
three  distinct  standpoints,  —  (1)  Mass  or  Weight;  (2)  Size; 
(3)  Brilliancy.  We  will  here  consider  the  brilliancy  alone, 


A   STAR-SPANGLED   BANNER  105 

as  comparatively  little  is  at  present  known  of  the  other  two 
factors. 

It  must  be  remembered  that  stars  which  now  give  out  little 
or  no  radiant  energy  are  not  necessarily  small.  Some  of  the 
largest  suns  in  the  Universe  are  feeble  from  extreme  old  age, 
while  others  are  dead  and  cold,  waiting  patiently  for  the  resur- 
rection that  comes  through  collision. 

If  the  stars  were  all  at  the  same  distance  from  us,  their  actual 
brilliancy  would  be  proportionate  to  their  apparent  magnitudes, 
and  could  therefore  be  easily  found.  On  the  other  hand,  if 
they  were  alike  in  actual  brilliancy,  we  could  estimate  their 
distances  from  us  by  that  alone. 

As  it  is,  however,  with  varying  distances  and  sizes,  the  prob- 
lem is  a  difficult  one,  and  will  probably  never  be  solved  except 
in  the  case  of  the  nearer  stars.  I  can  give  here  only  a  few 
illustrations  of  what  has  been  ascertained  concerning  the  radiant 
energy  of  the  stars. 

Let  us  first  consider  what  we  may  reasonably  expect  to  find, 
and  then  compare  our  expectations  with  the  results  actually 
attained. 

Take  a  certain  distance,  A  L,  and  divide  it  into  11  regularly 
increasing  intervals.  Call  these  shorter  distances  B,  C,  D,  etc. 

Let  us  suppose  that  we  are  at  A,  and  that  there  are  10  stars 
at  the  distance  B.  The  actual  brilliancy  of  these  stars  we  may 
suppose  to  vary  regularly,  so  that  (from  A)  the  largest  appears 
equal  to  Sirius,  and  the  smallest  is  just  visible.  If  there  be  a 
similar  set  at  each  of  the  other  distances,  only  9  of  those  at  C 
will  be  visible.  Of  those  at  D  only  8  will  be  seen.  And  so  OD. 
Those  which  become  invisible  will  of  course  be  the  smallest, 
but,  as  the  others  will  appear  smaller  on  account  of  the  greater 
distance,  the  effect  will  be  as  though  the  largest  were  dropped. 
Beyond  the  distance  K  all  will  be  invisible.  Out  of  the  110 
stars  only  55  (one  half)  will  be  visible  to  us.  We  may  expect, 
therefore,  to  find  all  sizes  of  stars  among  those  nearest  to  us, 
while  at  the  limit  of  visibility  only  the  very  largest  will  be 
visible,  and  they  will  appear  very  faint.  For  every  star  visible 


106     HOW   TO   KNOW   THE   STARRY   HEAVENS 

there  will  be  at  least  one  invisible.  This  reasoning  may  be 
applied  to  telescopic  stars  as  well  as  to  visible  ones. 

First  Step.  —  It  is  found,  however,  that  there  are  no  very 
large  stars  among  those  which  are  near  to  our  System.  The 
largest,  both  actually  and  apparently,  is  Sirius,  which  equals 
about  30  suns  like  ours.  Procyon,  Altair,  and  Alpha  Centauri 
are  also  brighter  than  our  Sun.  The  fifth-magnitude  star  known 
as  61  Cygni  is  small,  and  some  of  the  binaries  and  multiple 
systems  contain  stars  which  give  out  hundreds  and  thousands 
of  times  less  light  than  our  Sun. 

Second  Step.  —  If  we  examine  those  which  are  very  much 
farther  away,  but  are  still  at  a  measurable  distance  from  us,  we 
find  stars  which  have  much  more  actual  brilliancy  than  any  of 
the  nearest  stars.  And  some  of  them,  in  spite  of  their  great 
distances,  are  equal  in  apparent  brilliancy  to  any  of  the  nearest 
stars,  with  the  single  exception  of  Sirius.  Those  known  as 
Arcturus,  Eegulus,  Antares,  and  Gamma  Cassiopeise,  are  each  of 
them  about  equal  in  brilliancy  to  1,000  suns  like  ours,  rolled 
into  one  huge  globe. 

Third  Step.  —  Among  those  which  are  at  absolutely  immeas- 
urable distances  from  us  we  should  not  expect  to  find  any  stars 
equal  in  apparent  magnitude  to  those  which  are  so  much  closer 
to  us.  But  here  we  meet  with  another  surprise.  For  some  of 
them  are  among  the  brightest  stars  in  the  heavens.  They  in- 
clude, indeed,  the  second  brightest  star  in  the  sky.  Rigel,  Can- 
opus,  and  Arided  are  each  of  them  many  thousands  of  times 
larger  and  more  brilliant  than  our  Sun.  In  fact  they  are  so 
large  and  brilliant  that  they  exceed  our  Sun  as  much  as  it 
exceeds  the  planets  which  yield  to  its  authority.  They  are  so 
enormous  that  the  mind  cannot  grasp  their  immensity.  They 
may  be  the  centres  of  systems  of  a  higher  order  than  ours,  with 
mighty  suns  for  planets,  huge  planets  for  satellites,  and  perhaps 
secondary  satellites  revolving  around  the  primary  ones. 

Yet,  for  all  we  know  to  the  contrary,  some  of  the  faint  stars 
that  fill  the  back-space  of  sky  may  exceed  them  as  much  as 
they  exceed  our  Sun. 


A   STAR-SPANGLED  BANNER  107 

We  have  now  taken  three  huge  steps  into  the  visible  Uni- 
verse, and  at  each  step  have  found  larger  suns  than  we  had 
come  across  before.  And  the  fact  that  some  of  those  that  are 
at  an  immeasurably  great  distance  from  us  are  quite  brilliant 
when  seen  from  the  Earth  shows  that  several  more  such  steps 
will  have  to  be  taken  before  we  get  beyond  the  great  mass  of 
fairly  visible  stars.  This  is  true  of  those  seen  without  the  aid 
of  the  telescope,  and  it  is  much  more  true  of  telescopic  stars. 

DARK  SUNS  AND  PLANETS 

It  has  been  already  mentioned  that  some  stars  are  not  now 
luminous,  their  heat  having  radiated  into  space  and  left  them 
dark  and  cold.  There  is  some  reason  to  believe  that  the  Uni- 
verse is  crowded  with  these  dead  and  dying  suns,  and  that  they 
are  in  fact  vastly  more  numerous  than  the  living  ones.  Besides 
these,  there  are  the  planets  which  probably  swarm  around  each 
and  every  star,  living  and  dead.  Our  own  star  has  countless 
millions  of  such  bodies  revolving  around  it.  Probably  about  a 
thousand  of  these  are  large  enough  and  near  enough  to  be  de- 
tected by  our  telescopes,  the  smallest  one  visible  (a  satellite  of 
Mars)  being  about  seven  miles  thick.  All  the  rest  are  too 
small  or  too  distant  to  be  seen  by  us.  The  only  evidence  we 
have  of  their  existence  is  that  they  occasionally  run  up  against 
our  atmosphere,  and  are  turned  to  flaming  gas  by  the  friction. 
Sometimes  fragments  of  them  reach  the  Earth's  surface  in  a 
solid  state,  unaltered  except  at  the  original  surface,  which  is 
vitrified  by  the  heat. 

VARIABLE   STARS 

While  sorting  out  the  stars  according  to  their  magnitudes, 
astronomers  have  discovered  that  large  numbers  of  them  vary 
in  brightness  at  different  times.  They  are  therefore  called 
"  variables,"  to  distinguish  them  from  those  which  give  out  a 
steady  unfluctuating  light. 


108     HOW   TO   KNOW   THE   STARRY   HEAVENS 

REGULAR  (SHORT-PERIOD)  VARIABLES 

Algol  Variables.  —  Some  of  these  variables  are  periodic  and 
regular  in  their  changes.  For  example,  Algol,  which  is  usually 
of  the  2.3  magnitude,  fades  every  three  days  to  the  3.5  magni- 
tude, and  recovers  its  normal  brightness  in  a  few  hours.  The 
spectroscope  shows  that  between  these  minima  it  is  alternately 
approaching  and  receding  from  us. 

It  is  evident  that  in  this  case  there  are  two  stars  revolving 
around  their  common  centre  of  gravity,  but  that  one  of  them  is 
smaller  and  gives  out  little  or  no  light.  When  it  passes  be- 
tween us  and  Algol  it  partly  eclipses  the  latter. 

The  period  of  revolution  (three  days),  and  the  speed  at  which 
Algol  moves  to  and  from  us,  show  that  they  are  about  3,000,000 
miles  apart.  The  invisible  companion  appears  to  be  about  the 
size  of  our  Sun,  and  Algol  larger.  But  their  masses,  taken  to- 
gether, are  probably  less  than  half  that  of  our  Sun  alone.1 

Speetroscopic  Double  Stars.  —  Beta  Lyrae  is  also  a  periodic 
variable,  with  partial  eclipses  every  6^  days.  But  in  this  case 
only  the  alternate  minima  are  equal.  The  spectroscope  shows 
the  usual  spectral  lines  to  be  doubled.  There  appear  to  be  two 
unequal  self-luminous  stars  revolving  around  each  other,  about 
30,000,000  miles  apart. 

It  is  obvious  that  these  two  classes  of  variables,  which  are 
both  regular  in  their  fluctuations,  are  only  apparently  variable, 
through  change  of  position.  Their  actual  brilliancy  remains 
the  same.  Most  of  them  are  white  stars. 

Stars  below  a  certain  magnitude  cannot  be  satisfactorily  ex- 
amined spectroscopically,  on  account  of  the  faintness  of  their 
spectra.  Among  the  stars  which  have  been  so  examined,  about 
one  in  seven  shows  some  inequality  of  radial  motion,  due  to 
large  invisible  companions. 

IRREGULAR   (LONG-PERIOD)   VARIABLES 

Mir  a  Ceti  Variables.  —  Other  variable  stars  (which  are 
generally  red)  differ  from  the  above  in  being  irregular,  both  as 

1  One  variable  of  this  type  has  a  period  of  only  4  hours. 


FIG.   59.  —  COLOURED  DOUBLE  STARS 


A   STAR-SPANGLED  BANNER  109 

to  time  and  brilliancy  of  maxima  and  of  minima.  Mira  Ceti, 
for  example,  has  a  maximum  varying  from  the  second  to  the 
fifth  magnitude,  and  a  minimum  varying  from  the  eighth  to 
the  ninth  magnitude.  Its  period  also  varies  irregularly, -being 
usually  about  eleven  months. 

The  cause  of  such  irregular  fluctuations  must  be  violent 
physical  and  chemical  reactions  such  as  take  place  on  all  large 
cooling  bodies  at  certain  critical  temperatures.  Even  our  Earth 
has  passed  through  such  crises  in  the  past,  and  our  Sun  has 
slight  spasms  every  eleven  years  from  the  same  cause.  Some 
day,  when  chemical  compounds  are  formed  on  the  cooling  sur- 
face of  the  Sun,  these  fluctuations  will  be  vastly  greater  than 
they  are  now. 

NEW  STARS 

"  Vague  on  the  night  the  mist  we  mark 

That  tells  where  met  the  random  suns."  —  G.  Sterling. 

Allied  to  these  irregular  long-period  variables  are  the  new 
stars  which  occasionally  flash  out  in  a  hitherto  vacant  part  of 
the  sky,  attain  a  maximum  brilliancy  in  a  few  days,  and  then 
slowly  die  out  again.  These  either  disappear  altogether  or  re- 
main as  telescopic  stars.  The  New  Star  in  Perseus  is  a  familiar 
example  of  this  class.  Inside  of  five  days  it  increased  from 
telescopic  invisibility  to  be  the  brightest  star  in  the  Northern 
Hemisphere  (magnitude  0).  It  then  faded  gradually  away,  and 
in  a  few  months  became  invisible  to  the  naked  eye. 

While  this  star  was  increasing  in  brilliancy,  the  spectroscope 
showed  an  almost  continuous  spectrum,  like  that  of  other  stars 
(Class  III).  But  bright  and  dark  bands  gradually  appeared 
in  it  (Class  IV),  and  it  ended  by  resembling  the  spectra  of 
the  true  nebulae  (Class  II). 

This  star's  sudden  appearance  was  not  due  to  its  coming 
toward  us  out  of  the  depths  of  space.  It  was  probably  caused 
by  a  sudden  and  stupendous  discharge  of  light  and  heat  from 
a  previously  existing  star  or  stars.  Such  an  outburst  may  be 
produced  in  several  ways. 


ilO     HOW  TO   KNOW  THE   STARRY  HEAVENS 

For  example :  (1)  two  stars  or  planets  may  come  into  more 
or  less  direct  collision,  and  spread  out  rapidly  into  a  huge 
nebula  of  flaming  gas.  This  will  cool  down  in  a  short  time  to 
the  temperature  of  space.  Or  (2)  the  colliding  stars  may  strike 
a  glancing  blow  and  cut  slices  out  of  each  other.  In  this  case 
the  slices  alone  will  turn  to  gas,  while  the  wounded  stars  will 
enter  into  partnership,  revolving  around  their  common  centre 
of  gravity.  Or  (3),  the  stars  may  not  strike,  but  pass  so  close 
to  one  another  that  their  solid  crusts  are  rent  asunder  by  tidal 
action,  exposing  the  molten  interior  of  one  or  both  of  them. 
Or  (4),  a  dark  star  may,  in  the  course  of  its  wanderings,  dash 
through  an  immense  swarm  of  "pocket  planets,"  commonly 
known  as  meteoric  bodies.  Such  a  collision  will  turn  the 
meteorites  into  gas,  and,  if  they  are  numerous  enough,  their 
dissipation  will  cause  a  temporary  brilliancy  like  that  observed. 
Or  (5),  a  single  cooling  body  may  reach  one  of  its  "  critical " 
periods,  and  flare  up  from  sudden  physical  or  chemical  reactions, 
like  those  attributed  to  Mira  Ceti,  only  more  severe. 

Collisions  and  minor  interferences  between  all  kinds  of 
heavenly  bodies  must,  in  the  very  nature  of  things,  take  place 
now  and  then,  seeing  what  multitudes  of  suns  and  worlds  are 
hurtling  through  space,  in  all  directions,  without  any  one  on 
board  to  guide  them.  And  at  the  same  time  collisions  appear 
to  be  necessary  to  prevent  the  Universe  from  dying  of  old  age. 
The  dead  or  dying  hulks  collide,  turn  to  gaseous  nebulae,  and 
start  afresh  to  form  new  systems  of  suns  and  worlds. 

DOUBLE  AND   MULTIPLE   STARS 

"  Those  double  stars 
Whereof  the  one  more  bright 
Is  circled  by  the  other." —  Tennyson. 

In  dealing  with  periodically  variable  stars  it  was  shown  that 
they  are  really  double-star  systems  with  orbits  turned  edgeways 
to  us.  There  are  immense  numbers  of  similar  systems  which 
do  not  happen  to  be  so  situated,  and  are  therefore  not  recognis- 
able as  "  doubles  "  by  the  spectroscope.  Some  of  them,  however, 


A  STAR-SPANGLED   BANNER  111 

are  so  near  to  us,  or  have  such  large  orbits,  that  the  telescope 
itself  enables  us  to  detect  their  "duplicity"  and  follow  their 
motions.  There  are  over  10,000  double  stars  now  known.  In 
about  fifty  cases  the  time  of  mutual  revolution  has  been  deter- 
mined with  some  certainty.  The  periods  vary  from  5.7  years1 
(the  shortest  telescopically  visible)  to  1,500  years.  Many  bi- 
naries take  thousands  of  years  to  complete  a  revolution,  and 
their  motions  have  not  been  followed  long  enough  to  fix  their 
orbits  and  periods. 

Those  dual  systems  of  suns  which  started  as  partners  not 
by  direct  collision,  but  by  tidal  division,  are  twins,  their  gase- 
ous contents  having,  at  first,  the  same  -heat  and  condition.  It 
takes  longer  for  a  large  mass  to  condense  into  a  sun,  and  then 
cool  off  and  die  of  old  age,  than  it  does  for  a  smaller  one.  There- 
fore the  larger  of  the  two  bodies  is  relatively  younger  than  its 
companion,  and  the  centre  of  brilliancy  of  its  spectrum  should 
be  nearer  to  the  violet  end.  Dual  systems  which  started  in 
other  ways  would  not  necessarily  have  this  spectral  peculiarity. 

For  some  unexplained  reason  there  is  much  more  variety  of 
colour  in  double  stars  than  in  single  ones.  They  are  sometimes 
of  a  green,  blue,  or  violet  colour  (see  Figure  59). 

In  some  cases  there  are  more  than  two  suns  in  the  same  sys- 
tem. The  North  Pole  Star,  for  example,  has  two  companions 
large  enough  to  be  classed  as  suns,  though  one  of  them  has 
ceased  to  give  out  any  appreciable  light  and  heat. 

There  appears  to  be  every  gradation  of  solar  systems  visible 
to  the  telescope.  They  vary  from  simple  ones  like  ours,  to 
highly  complex  star-clusters,  like  the  one  in  the  constellation 
of  Hercules.  This  has  over  6,000  visible  suns,  and  appears  to 
be  surrounded  by  long  spirally  radiating  wisps  of  nebulous  mat- 
ter in  which  other  stars  are  entangled. 

SOLAR  DRIFT. 

It  has  long  been  known  that  some  of  the  nearer  stars  are 
not  absolutely  fixed,  but  are  slowly  changing  their  positions  in 

l  Delta  Equulei. 


112     HOW  TO   KNOW  THE   STARRY   HEAVENS 

the  heavens.  And  the  spectroscope  has  told  us  that,  while  some 
stars  are  coming  nearer  to  us,  others  are  retreating.  From  the 
nature  of  the  evidence  we  may  safely  assume  that  all  stars  are 
in  motion,  and  that  our  Sun  partakes  in  this  stellar  drift, 
carrying  its  planets  along  with  it. 

If  our  sun  be  assumed  to  be  motionless,  then  the  observed 
motion  of  a  star  must  be  real,  due  to  its  own  drift  through  space. 
But  if  our  System  is  also  adrift,  the  observed  motion  of  a  star 
may  be  only  apparent,  due  to  our  own  change  of  place.  Or  it 
may  be  partly  real  and  partly  apparent. 

It  is  obvious  that,  if  our  Sun  is  drifting  in  one  direction 
(carrying  us  along  with  it),  the  stars  toward  which  it  is  drifting 
will  appear  to  open  out  or  separate,  while  those  from  which  it  is 
retreating  will  appear  to  close  up.  At  the  same  time  those 
which  we  are  passing  will  appear  to  drift  backward. 

Now  the  telescope  shows  that  all  three  of  these  peculiarities 
are  actually  taking  place.  The  stars  around  Vega,  in  the  con- 
stellation of  the  Lyre,  are  gradually  separating  from  one  an- 
other, those  on  the  opposite  side  of  the  heavens  are  as  gradually 
closing  up,  and  those  at  right  angles  to  this  line  of  march  are 
drifting  backward. 

The  conclusion  is  obvious.  The  motions  of  the  stars  are,  at 
least  in  part,  only  apparent.  Our  whole  System  is  drifting  in 
the  direction  of  Vega. 

Lest  we  should  have  made  a  mistake,  let  us  inquire  of  the 
tele-spectroscope,  and  find  out  which  stars  are  approaching  us 
and  which  are  receding. 

According  to  the  spectroscope  and  spectrograph,  the  major- 
ity of  the  stars  about  Lyra  are  approaching  us  at  the  aver- 
age speed  of  about  12J  miles  a  second.  And  the  majority 
of  stars  opposite  that  constellation  are  receding  from  us  at 
about  the  same  rate.  In  the  circle  of  the  heavens  between 
these  two  points  there  is  no  decided  motion  either  to  or 
from  us. 

We  thus  see  that  two  absolutely  independent  sets  of  evidence 
both  point  to  the  same  conclusion.  Our  Sun  is  drifting  toward 


FIG.  60.  —  STAR- CLUSTER  IN  HERCULES 
Lick  photograph. 


A   STAR-SPANGLED   BANNER  113 

Vega  at  the  rate  of  12|  miles  a  second,  and  all  the  planets  share 
in  the  "  solar  drift." 

On  account  of  this  solar  drift,  the  orbit  of  the  Earth,  in- 
stead of  being  an  ellipse,  is  really  of  a  corkscrew  shape,  the 
axis  of  the  "  corkscrew  "  being  in  the  direction  of  the  constel- 
lation Lyra.1 

STELLAR  DRIFT 

After  allowing  for  the  apparent  motions  of  the  stars,  due  to 
our  own  drift,  there  remain  certain  real  motions  of  the  stars, 
due  to  their  own  drift. 

For  instance,  the  telescope  shows  that  the  star  known  as 
1,830  Groombridge  is  moving  steadily  along  at  the  rate  of  over 
200  miles  a  second.  Some  other  stars  have  motions  as  real, 
though  not  as  rapid.  The  spectroscope  shows  that  some  stars 
are  actually  approaching  us,  while  others  are  as  actually  reced- 
ing from  us. 

In  some  cases  a  number  of  stars  are  drifting  along  together, 
showing  that  they  form  a  family  group.  The  Pleiades  form 
a  familiar  example  of  this  social  drift  through  space. 

1  The  Cluster  of  Hercules  is  not  very  far  away  from  the  part  of  the  sky  which 
we  are  approaching.  It  is  possible  that  our  System  may  form  a  distant  part  of 
one  of  its  encircling  wisps  of  star-strewn  nebulous  matter.  In  this  case  we  may 
eventually  be  drawn  into  the  cluster. 


CHAPTER  X 

CONSTRUCTION   OF  THE  UNIVERSE 

"Secondly,  .  .  .  what  is  the  arrangement  of  the  stars  in  space?  Especially, 
what  is  the  relation  of  the  Galaxy  to  the  other  stars  ?  In  what  senses,  if  any, 
can  the  stars  be  said  to  form  a  permanent  system  ?  Do  the  stars  which  form  the 
Milky  Way  belong  to  a  different  system  from  the  other  stars,  or  are  the  latter  a 
part  of  one  universal  system  ? "  —  Prof.  Simon  Newcomb. 

DISTRIBUTION  OF  STARS 

IN  the  stellar  population  of  the  visible  Universe  there  are 
very  great  differences  of  distribution.  Although  it  would 
be  difficult  to  point  a  powerful  telescope  to  any  part  of  the  sky 
and  find  the  field  of  view  absolutely  hare  of  stars  or  nebulae, 
yet  in  some  directions  they  are  very  much  more  scarce  than  in 
others.  In  fact  the  visible  inhabitants  of  space  are  as  unevenly 
distributed  as  is  the  human  population  of  Usona.  There  are 
belts  of  sky  that  are  swarming  with  cities,  towns,  and  villages 
of  social  stars,  and  have  the  intervening  spaces  well  peopled 
with  more  independent  families.  And  there  are  large  tracts 
of  sky  that  are  comparatively  thinly  inhabited  with  visible  suns. 
Some  of  the  star-clusters  are  almost  as  crowded  as  are  our  cities. 
In  some  of  the  thickly  settled  tracts  nearly  all  the  stars  are  well 
developed,  while  in  others  large  numbers  of  the  inhabitants  are 
in  a  nebulous  embryonic  stage. 

ALL  SORTS  AND  CONDITIONS  OF  SUNS 

In  that  part  of  the  Universe  which  is  at  present  visible  from 
our  Earth,  there  are  many  different  structures  and  styles  of 
architecture.  And  all  stages  of  construction  and  development 
are  represented.  There  are  systems  that  appear  to  be  still  in 
the  throes  of  birth;  systems  in  young  and  vigorous  growth; 


CONSTRUCTION   OF  THE    UNIVERSE          115 

systems  in  the  strength  and  pride  of  maturity ;  systems  that  are 
tottering  toward  the  grave ;  and  systems  that  are  cold  in  death 
and  waiting  for  the  resurrection  that  will  surely  come. 

THE  CONSTELLATIONS 

The  stars  that  are  visible  to  the  naked  eye  have  long  been 
divided  up  by  man,  for  his  own  convenience,  instruction,  and 
amusement,  into  groups  or  constellations.  About  48  of  these 
are  extremely  ancient,  and  the  rest  are  quite  modern. 

The  ancient  constellations  appear  to  have  had  a  priestly  origin 
and  an  important  religious  significance.  The  stars  were  divided 
into  groups,  each  of  which  was  supposed  to  form  a  picture  of 
some  person,  animal,  or  inanimate  object.  The  48  constella- 
tions contained  54  figures,  which  formed  a  huge  sky-picture 
whose  mystic  meaning  has  long  been  lost  sight  of  by  the 
multitude. 

These  fanciful  figures  appear  to  have  been  invented  by  a 
people  who  lived  south  or  southeast  of  the  Caspian  Sea,  nearly 
5,000  years  ago.  As  the  stars  near  the  South  Pole  were  invisi- 
ble from  their  part  of  the  world,  they  naturally  left  that  section 
of  the  sky  unfigured.  The  size  of  the  circle  which  was  left 
blank  shows  us  that  the  constellation-makers  lived  about  39° 
north  of  %the  Equator.  And  the  animals,  etc.,  which  they  pic- 
tured in  the  sky,  tell  us  their  longitude  on  the  earth ;  for  they 
naturally  depicted  the  animals  with  which  they  were  acquainted, 
and  none  others.  Even  the  animal  monstrosities  were  combi- 
nations of  familiar  animals. 

Owing  to  what  is  known  as  the  Precession  of  the  Equinoxes 
(described  in  Chapter  XII),  this  circular  blank  space  does  not 
now  centre  at  the  South  Pole,  and  the  amount  of  its  drift  tells 
us  that  the  constellations  were  completed  about  2800  B.  c. 

The  first  groups  of  stars  to  be  pictured  out  were  evidently  the 
twelve  Signs  of  the  Zodiac,  likewise  known  as  the  "Mansions 
of  the  Sun."  The  picture-makers  were, astronomers  of  no  mean 
ability.  They  had  already  determined  the  length  of  the  year 


116     HOW  TO   KNOW   THE   STARRY   HEAVENS 

with  some  degree  of  accuracy.  They  had  recognised  the  fact 
that  the  stars  are  in  the  sky  during  the  daytime,  though  they 
cannot  then  he  seen  under  ordinary  circumstances.  They  had 
even  traced  out  the  annual  path  of  the  Sun  among  the  stars. 
This  was  a  remarkable  achievement  which  few  of  us  would  have 
the  patience  and  ingenuity  to  accomplish  unaided.  To  do  it  they 
must  have  rigged  up  some  sort  of  equatorial  mounting,  with 
a  number  of  pointers  directed  to  the  most  prominent  stars. 

To  enable  them  to  ascertain  the  Sun's  position  in  this  path, 
or  Ecliptic,  all  the  year  round,  they  divided  up  the  neighbouring 
stars  into  twelve  constellations.  This  made  it  easier  to  keep 
track  of  the  months,  seasons,  and  years.  It  also  enabled  them 
to  find  the  four  critical  days  in  which  the  seasons  culminated. 
Being  Sun-worshippers,  like  nearly  all  the  nations  of  antiquity, 
they  were  very  anxious  to  ascertain  the  exact  date  of  the  sab- 
baths, new  moons,  equinoxes,  and  solstices ;  for  otherwise  they 
could  not  time  their  feasts,  fasts,  and  sacrifices  so  as  to  have 
them  credited  in  the  heavenly  ledger.  They  regarded  the  Sun 
as  a  Great  Spirit  labouring  for  the  good  of  mankind,  and  fight- 
ing the  powers  of  cold  and  darkness  with  varying  success.  Each 
summer  he  got  the  upper  hand,  but  in  the  winter  he  lost  his 
strength,  like  his  Semitic  prototype,  Samson,  when  he  was  shorn 
of  his  flowing  locks.  As  the  very  existence  of  the  human  race 
depended  on  the  Sun-God's  success,  the  struggle  was  watched 
with  never-flagging  interest  and  anxiety. 

The  year  was  then  reckoned  to  begin  in  the  spring,  at  the 
time  when  the  days  and  nights  were  equal.  The  Sun-God  was 
then  in  the  centre  of  a  group  of  stars  which  they  pictured  out 
as  a  Bull  (Taurus)}-  At  the  longest  day  their  God  was  in  the 
centre  of  a  group  which  they  named  the  Lion  (Leo).  At  the 
autumn  equinox  he  was  in  the  centre  of  a  group  which  they 
imagined  to  represent  a  Scorpion  (Scorpio).  And  on  the  short- 
est day  in  winter  he  was  in  the  midst  of  a  group  which  they 
called  the  Water- Carrier  (Aquarius). 

1  Subsequent  generations  long  inherited  the  tradition  that  each  year  was  opened 
by  a  bull  with  golden  horns. 


FIG.   63.  —  SPIRAL  NEBULA  IN  TRIANGULUM 
Lick  photograph. 


V\BRT7> 
Or  TME 

UNIVERSITY 

or 


CONSTRUCTION   OF  THE   UNIVERSE          117 

These  significant  symbols  became  famous  in  mythology,  and 
are  several  times  mentioned  in  the  Hebrew  Bible  and  early 
Christian  writings.  The  brightest  star  in  or  near  each  of  the 
four  groups  has  been  known  ever  since  as  a  "  royal "  star,  though 
its  significance  was  lost  when  the  Precession  of  the  Equinoxes 
dethroned  the  celestial  Bull  and  set  the  heavenly  Ram  in  the 
place  of  honour.  These  royal  stars  were  Aldebaran,  Eegulus 
Antares,  and  Fomalhaut. 

The  Zodiacal  groups  having  been  satisfactorily  marked  out, 
the  northern  stars  were  made  into  a  great  winged  Dragon 
(Draco),  which  was  supposed  to  guard  the  pole  of  the  heavens.1 
The  rest  of  the  constellations  were  then  figured  off  in  other 
parts  of  the  sky.  They  were  all  connected  together,  each  figure 
forming  a  portion  of  the  same  great  mythological  sky  picture. 
As  a  rule  they  were  all  upright  when  on  the  meridian,  either 
north  or  south.2 

The  picture-makers  of  2800  B.  a,  looking  south  at  midnight 
at  the  spring  equinox,  imagined  a  huge  man  in  the  sky,  crush- 
ing a  scorpion  with  his  left  foot,  and  strangling  an  enormous 
serpent  which  was  coiled  around  his  body.  The  same  observers, 
on  turning  to  the  north,  pictured  a  second  man,  kneeling  on  one 
knee,  and  pressing  the  head  of  a  winged  dragon  with  the  other 
foot.3  The  stars  which  were  faintly  visible  above  the  southern 
horizon  were  afterward  worked  into  the  picture,  and  the  knowl- 
edge of  it  was  handed  down,  from  generation  to  generation,  as 
a  divine  revelation,  to  change  or  add  to  which  would  be  sacrilege. 

These  picture-makers  appear  to  have  domesticated  cattle, 
sheep,  goats,  dogs,  and  horses.  They  hunted  bears,  lions,  and 

1  In  after  years  the  Precession  of  the  Equinoxes  made  this  winged  dragon  slip 
off  the  pole  of  the  heavens.     He  was  then  said  to  have  been  overcome  by  the 
stalwart  Michael  and  thrown  into  a  pit  that  had  no  bottom.     One  rather  lurid 
writer  tells  us  that  "his  tail  drew  the  third  part  of  the  stars  of  heaven,  and  did 
cast  them  to  the  Earth." 

2  See  the  Constellation  Chart  at  the  end  of  this  book. 

8  The  head  of  this  winged  dragon  was  then  turned  threateningly  toward  the 
man,  and  had  two  bright  stars  for  eyes.  Its  wings  have  since  then  been  cut  off, 
in  order  to  make  room  for  other  constellations.  Its  head  has  also  been  turned 
around,  for  the  same  reason,  so  that  its  fierce  basilisk  stare  has  been  lost. 


118     HOW  TO   KNOW  THE   STARRY  HEAVENS 

hares,  using  bows  and  arrows,  as  well  as  spears.  They  do  not 
appear  to  have  been  acquainted  with  the  tiger,  elephant,  camel, 
hippopotamus,  or  crocodile.  To  their  north  lay  a  sea,  and  they 
were  familiar  with  ships  and  with  sea-monsters.  They  sacri- 
ficed on  altars ;  knew  the  stories  of  the  Fall  and  of  the  Deluge, 
and  probably  devised  many  of  the  constellations  to  keep  record 
of  them. 

These  constellations  became  known  to  the  Greeks.  They  are 
described  in  a  poem  of  Aratus  (260  B.  c.),  and  are  mentioned  in 
the  star-catalogue  of  Ptolemy  (150  A.  D.).  In  modern  times 
their  religious  significance  has  been  lost.  From  time  to  time 
their  irregular  and  arbitrary  boundaries  have  been  changed,  and 
a  number  of  other  groups  have  been  formed,  to  fill  in  the  vacant 
places.  About  90  constellations  are  now  recognised.1 

Most  of  the  larger  stars  are  known  by  names  which  were 
given  them  by  the  Arab  astronomers.  Large  numbers  are  dis- 
tinguished by  the  genitive  form  of  the  name  of  the  constella- 
tion in  which  they  occur,  with  a  Greek  letter  prefixed  to  it. 
Many  thousands  are  known  only  by  numbers  in  certain  star- 
catalogues.  The  great  majority  of  telescopic  stars  have  no 
names  at  all. 

It  is  not  necessary  to  describe  the  constellations  here,  as  they 
are  best  studied  by  night,  under  the  blue  heavens,  with  an  oc- 
casional reference  to  a  star  atlas.  The  four  star  charts  given 
at  the  end  of  the  book  will  be  a  help  to  beginners  who  live 
in  the  Northern  Hemisphere.  One  of  them  gives  the  North 
Polar  heavens,  and  the  other  three  include  all  the  Equatorial 
constellations.  In  order  to  avoid  confusion,  no  names  or  di- 
visions are  marked  on  these  charts,  but  each  one  is  accom- 
panied by  a  key,  giving  all  necessary  particulars. 

The  first  of  the  Equatorial  Charts  is  intended  to  be  used  in 
the  spring  (from  December  to  March),  the  second  in  the  sum- 

1  Those  interested  in  the  ancient  constellations  should  read  an  article  by  the 
late  Richard  A.  Proctor,  in  his  "Myths  and  Marvels  of  Astronomy,"  and  another 
by  E.  W.  Maunder  (F.E.A.S.),  entitled  "The  Oldest  Picture  Book  of  All,"  in 
the  "Nineteenth  Century  Magazine"  for  September,  1900. 


FIG.   64.  —  THE  GREAT  NEBULA  IN   ANDROMEDA 


CONSTRUCTION   OF  THE   UNIVERSE          119 

mer  (from  April  to  July),  and  the  third  in  the  autumn  (from 
August  to  November).  The  Polar  Chart  can  be  used  all  the 
year  round. 

In  order  to  understand  these  charts,  the  learner  should  go 
outside  on  a  clear  starlight  evening,  about  nine  o'clock,  and 
seat  himself  in  a  rocking-chair,  facing  the  south.  If  he  can  tilt 
the  chair  back  against  some  trustworthy  support,  it  will  be  an 
advantage,  as  he  may  otherwise  see  the  wrong  kind  of  stars. 
A  dark  lantern  will  be  an  assistance,  to  enable  him  to  see  the 
charts  and  keys  at  intervals. 

Let  the  beginner  now  select  the  Equatorial  Chart  which  is 
suitable  for  the  time  of  the  year.  At  the  foot  of  the  chart 
he  will  find  a  number  of  dates.  Selecting  the  date  which  comes 
nearest  to  that  of  his  observation,  he  will  find  that  the  stars 
represented  above  it  agree  with  those  in  the  heavens  in  front  of 
him,  from  the  horizon  to  the  point  overhead. 

When  the  beginner  wishes  to  identify  the  stars  which  are 
farther  north  than  the  point  overhead,  he  should  take  the  Polar 
Chart  and  turn  it  till  the  proper  date  is  at  the  foot  of  the  chart. 
He  can  then  lean  well  back  in  his  chair,  when  he  will  be  able 
to  recognise  the  resemblance  between  the  brightest  of  the  stars 
above  him  and  those  represented  on  the  chart.  With  the  help 
of  the  proper  key  the  names  of  the  stars  and  constellations  can 
be  gradually  and  pleasantly  acquired. 

A  good  way  to  make  a  beginning  is  to  get  acquainted  with 
the  names  and  positions  of  the  brighter  stars  on  clear  evenings, 
to  divide  them  up  into  squares,  triangles,  etc.,  and  then  to  fill 
in  with  the  fainter  ones.  After  having  done  this  in  the  Polar 
regions,  they  can  be  connected  with  stars  farther  south,  giving 
particular  attention  to  those  on  or  near  the  Equator  and  Ecliptic.1 

1  The  Equator  can  easily  be  found  by  fastening  a  stick  or  pointer  to  the  top 
of  a  fence,  so  that  it  points  to  the  northwestern  star  in  the  Band  of  Orion,  when 
it  is  due  south.  All  the  Equatorial  stars  will  occupy  the  same  position  when 
they  come  to  the  local  meridian.  The  Ecliptic  can  easily  be  found  (approximately) 
by  noting  the  position  of  the  Moon  among  the  stars  every  night  that  it  is  visible. 
The  large  planets  are  also  on  or  near  the  Ecliptic.  To  prevent  their  being  mis- 
taken for  stars  it  should  be  remembered  that  they  do  not  twinkle,  but  shine  with 


120     HOW   TO   KNOW   THE   STARRY   HEAVENS 

In  the  course  of  a  year  the  greater  part  of  the  heavens  will 
thus  come  under  observation  without  the  necessity  of  staying 
up  late  at  night.1 

Most  of  the  stars  in  these  constellations  are  only  visually 
connected,  so  that  an  observer  in  a  distant  solar  system  would 
find  them  altogether  differently  arranged.  There  are  many 
natural  groupings,  however,  like  that  known  as  the  Pleiades, 
and  another  including  several  of  the  brighter  stars  in  the  con- 
stellation of  the  Great  Bear.  Such  natural  groups  are  being 
discovered  by  the  fact  that  the  stars  composing  them  are  drift- 
ing in  the  same  direction  and  at  the  same  speed. 

Most  of  the  telescopic  star-clusters  are  evidently  family 
groups,  as  are  also  the  thousands  of  double,  treble,  and  quad- 
ruple stars  which  revolve  around  their  common  centres  of 
gravity. 

THE  GALAXY 

"  A  broad  and  ample  road  whose  dust  is  gold, 
And  pavement  stars."  —  Milton. 

The  largest  and  most  important  structure  in  the  visible  Uni- 
verse is  that  nebulous  haze  of  invisible  suns  which  is  known  as 
the  Via  Lactea,  the  Galaxy,  or  the  Milky  Way.  In  fact,  if  the 
Universe  were  no  larger  than  the  visible  part  of  it,  we  might  al- 
most say  that  the  Universe  itself  consisted  of  the  Milky  Way 
and  its  appendages. 

With  the  naked  eye  the  Galaxy  forms  the  most  conspicuous 
object  of  the  midnight  heavens.  It  is  an  irregular  hazy  ring 
crossing  the  celestial  sphere  in  a  great  circle.  This  circle  is 
like  that  containing  the  Signs  of  the  Zodiac,  but  is  very  much 
more  tipped  up  with  reference  to  the  equator.  A  telescope  of 

a  steady  unflinching  light.  It  will  soon  be  found  that  they  gradually  change 
their  positions  among  the  stars  that  lie  near  the  Ecliptic. 

1  "Astronomy  without  a  Telescope,"  by  E.  W.  Maunder,  F.R.A.S.,  will  be 
found  to  make  the  subject  more  interesting.  This  may  be  followed  by  the  study 
of  "Astronomy  with  an  Opera  Glass,"  and  "The  Pleasures  of  the  Telescope," 
both  bv  Garrett  P.  Serviss. 


CONSTRUCTION   OF  THE   UNIVERSE 

moderate  power  shows  that  it  consists  of  a  "blinding  snow- 
storm "  of  suns  and  star-clusters. 

This  Milky  Way  was  formerly  considered  to  form  a  kind  of 
irregular  disc  or  "  grindstone,"  with  our  Solar  System  not  very 
far  from  its  centre.  More  accurate  observations,  however,  have 
caused  this  theory  to  be  discarded.  It  is  now  believed  to  con- 
sist of  long  spiral  wisps  or  serpent-like  streams  of  nebulous 
haze,  of  more  or  less  circular  section,  and  considerably  distorted 
by  projection.  In  one  place  the  main  stream  splits  in  two,  but 
finally  comes  together  again.  This  split  portion  has  a  number 
of  narrow  nebulous  channels  crossing  the  dark  interval  which 
separates  them,  and  dividing  it  into  irregular  islands,  or  "  coal- 
sacks."  If  we  were  on  one  side  of  this  Milky  Way,  instead  of 
being  inside  it,  we  should  probably  see  it  as  a  vast  ring-like  spiral 
nebula,  with  its  centre  of  rotation  still  comparatively  empty. 

If  the  observer  is  favourably  situated,  nearly  all  of  the  Milky 
Way  may  be  seen  from  one  .station  in  the  course  of  a  long 
winter  night.  Such  an  observation  will  soon  show  that,  al- 
though very  irregular,  these  "  flowing  robes  of  infinite  space  " 
form,  on  the  whole,  a  nearly  complete  girdle  around  our  part  of 
the  Universe. 

With  a  telescope  it  is  easily  seen  that  the  Galaxy  consists 
of  an  innumerable  host  of  telescopic  stars,  promiscuously  dis- 
tributed, with  thicker  clusters  or  aggregations  here  and  there 
(see  Figures  47,  48,  and  61).  Even  the  naked-eye  stars  (with 
the  exception  of  the  nearest)  are  found  to  be  most  numerous  in 
and  around  the  Milky  Way,  and  to  thin  out  gradually  as  we 
approach  its  poles  on  either  side.  The  same  is  true  of  the 
stellar  nebulae.  The  entire  visible  system  probably  resembles 
a  watch  in  shape,  slowly  rotating  on  its  axis,  and  condensing 
into  a  flat  spiral  disc. 

THE   NEBULA 

While  it  is  found  that  the  stars  and  stellar  nebulae  are  most 
numerous  near  the  Milky  Way,  it  is  rather  startling  to  find 
that  with  the  Gaseous  Nebulae  the  reverse  is  true.  There  are 


HOW  TO   KNOW  THE   STARRY   HEAVENS 

very  few  of  them  in  or  near  the  Milky  Way,  and  they  increase 
in  numbers  toward  its  poles. 

The  Magellanic  Clouds,  in  the  Southern  Hemisphere,  greatly 
resemble  the  Milky  Way  in  appearance ;  but,  instead  of  being 
composed  of  stars  and  stellar  nebulae,  they  consist  of  stars  and 
gaseous  nebulae. 

Some  nebulae  are  diffuse  and  sprawling,  as  though  the  central 
attraction  was  not  strong  enough  to  draw  them  together  or  had 
been  overpowered  by  some  outside  influence.  One  of  them  has 
been  traced  along  the  starry  sphere  for  a  length  of  8  degrees. 
Supposing  that  its  distance  from  us  is  only  two  million  times 
the  Sun's  distance  (and  it  can  hardly  be  less),  it  must  be  as 
long  as  from  here  to  the  nearest  star.  Though  many  of  the 
gaseous  nebulas  are  of  enormous  thickness,  they  all  appear  to 
be  transparent. 

The  late  Professor  Keeler,  a  short  time  before  his  death, 
found  that  the  telescopic  camera  showed  twenty  times  as  many 
nebulae  as  the  telescope  alone.  The  number  within  the  range 
of  the  photographic  telescope  is  now  put  at  120,000  or  more, 
and  the  majority  of  them  are  more  or  less  spiral,  like  those 
represented  in  Figures  63  and  85. 

The  Great  Nebula  in  Andromeda  has  a  condensed  nucleus 
(giving  a  continuous  spectrum)  surrounded  by  a  swirl  of  nebu- 
lous matter.  In  fact,  it  looks  not  unlike  the  planet  Saturn 
surrounded  by  its  rings  (see  Figure  64).  It  is,  however,  im- 
mensely larger  than  our  entire  Solar  System,  and  is  probably  a 
galaxy  of  suns. 

When  a  spiral  nebula  is  turned  edgeways  to  us,  it  is  not 
recognisable  as  a  spiral,  but  has  a  more  or  less  lenticular  shape 
(see  Figure  65).  The  remarkable  Ring  Nebula  in  Lyra  re- 
sembles in  telescopic  appearance  one  of  the  familiar  vortex 
rings  which  occasionally  rise  from  the  smoke-stack  of  a  steam- 
engine.  Like  most  of  the  ring-nebulae,  it  has  a  faint  star  in  its 
centre.  But  photographs  recently  taken  by  Professor  Schae- 
berli  at  Ann  Arbor,  Michigan,  show  that  a  two-branched  spiral 
originates  at  and  surrounds  this  central  star.  The  visible  nebu- 


FIG.  67.  —  THE  TRIFID  NEBULA  IN  SAGITTARIUS 

Composed  of  glowing  gas.     The  rifts  were  probably  caused  by  stars  drifting  through 
body  of  nebula. 


CONSTRUCTION   OF  THE   UNIVERSE 

lous  ring  is  the  most  prominent  part  of  this  double  spiral  (see 
Figure  66). 

Other  nebulae  have  very  irregular  outlines,  at  some  places  clean 
cut,  and  elsewhere  fading  gradually  away.  Some  are  split  up 
by  sharp  fractures,  as  though  they  had  been  torn  asunder  by 
some  wandering  star  drifting  through  them.  Such  is  the  Trifid 
Nebula  in  Sagittarius  (see  Figure  67).  The  Great  Nebula  in 
Orion  also  has  some  of  these  peculiarities,  and  has  several  nebu- 
lous stars  connected  with  it.  Its  present  irregular  outlines  may 
be  partly  due  to  the  disrupting  influence  of  these  stars,  or  it  is 
possible  that  the  neighbouring  nebula  may  have  collided  with 
it.  Seen  through  a  powerful  telescope,  it  forms  one  of  the  most 
magnificent  objects  in  the  heavens.  Its  real  size  is  so  enormous 
that  the  mind  cannot  realise  its  vastness.  It  has  been  estimated 
that  if  a  million  discs  as  large  as  the  orbit  of  Neptune  were 
placed  in  front  of  the  nebula,  they  would  not  be  sufficient  to 
hide  it  from  us.  The  spectroscope  shows  that  the  matter  com- 
posing it  is  in  the  form  of  gas  so  diffuse  as  to  be  almost  a 
vacuum  (see  Figure  68). 

It  was  formerly  supposed  that  all  nebulae  are  at  an  intense 
heat,  causing  the  peculiar  glow  which  makes  them  visible  to  us. 
It  seems  probable,  however,  that  the  more  diffuse  of  them  are 
at  the  temperature  of  space  ( — 230°  F.),  and  that  their  light  is 
an  electrical  phenomenon  due  to  a  rain  of  negatively  charged 
particles  driven  off  from  the  stars  (in  the  form  of  coronas)  by 
the  repulsive  action  of  light.  This  would  explain  the  simple 
nature  of  their  spectra,  as  the  glow  would  be  chiefly  confined  to 
the  surface,  where  the  lighter  gases,  like  hydrogen  and  helium, 
collect. 

UNSOLVABLE  PROBLEMS 

Although  a  great  many  facts  bearing  on  the  distribution  and 
relationships  of  the  visible  stars  and  nebulae  have  been  discov- 
ered in  the  last  few  years,  we  are  not  yet  in  a  position  to  give 
a  satisfactory  answer  to  the  questions  asked  in  the  quotation  at 
the  beginning  of  this  chapter.  At  present  we  can  only  say,  with 
the  late  Richard  A.  Proctor : 


124    HOW  TO   KNOW   THE   STARRY  HEAVENS 

"The  sidereal  system  is  altogether  more  complicated  and  more 
varied  in  structure  than  has  hitherto  been  supposed :  in  the  same 
region  of  the  stellar  depths  co-exist  stars  of  many  orders  of  real  mag- 
nitude ;  all  the  nebulse,  gaseous  or  stellar,  planetary,  ring-formed, 
elliptical,  and  spiral,  exist  within  the  limits  of  the  sidereal  system ; 
and,  lastly,  the  whole  system  is  alive  with  movements  the  laws  of 
which  may  one  day  be  recognised,  though  at  present  they  are  too  com- 
plex to  be  understood." 

The  Universe  is  so  vast  that  a  bird's-eye  view  of  it  cannot  be 
obtained  from  a  single  standpoint,  and  though  that  standpoint 
is  being  swept  along  through  the  lofty  corridors  at  a  speed  fifty 
times  as  great  as  that  of  a  cannon-ball,  the  life  of  the  human 
race  is  hardly  long  enough  to  profit  by  the  change  of  position. 
We  are  like  the  man  in  the  forest  who  said  that  he  could  not 
see  the  wood  for  the  trees.  Even  in  those  parts  where  we 
imagine  that  we  can  see  a  limit  to  the  celestial  forest,  we  can- 
not be  sure  that  it  is  not  a  mere  clearing  in  the  woods,  or  a  gap 
between  our  forest  and  the  next  one.  And  the  situation  is  com- 
plicated by  the  fact  that  the  celestial  trees  are  not  anchored  fast 
to  a  visible  ground,  but  are  drifting  around  in  all  directions, 
while  we  are  prisoned  on  a  conveyance  whose  movements  we 
are  utterly  unable  to  control. 

To  use  the  simile  of  Sir  Isaac  Newton,  we  can  learn  some- 
thing of  the  celestial  pebbles  that  lie  around  us,  but  the  great 
ocean  is  beyond.  The  finite  cannot  grasp  the  infinite,  nor  can 
the  ephemeron  comprehend  the  eternal.  And  if  it  could,  what 
then? 


FIG.  68.  —  GREAT  NEBULA  IN  ORION 
Lick  photograph. 

Irregular  mass  of  glowing  gases.     Many  millions  of  times  larger  than  Solar  System. 
Irregularities  possibly  due  to  collision  with  neighbouring  nebula. 


CHAPTER  XI 

SOLAR  ARCHITECTURE 

"We  know  the  Sun  to  be  infinitely  more  complex  in  structure  .  .  .  than  it 
was  formerly  supposed  to  be.  ...  We  have  learned  that  .  .  .  the  glowing  veil 
of  air  hides  by  day  .  .  .  the  largest  (though  not  the  most  massive)  part  of  that 
Sun."  — Richard  A.  Proctor. 

THE  individual  construction  of  the  stars  themselves  can 
best  be  found  by  studying  our  Sun  and  comparing  its 
peculiarities  with  those  of  more  distant  suns.     This  comparison 
has  led  to  the  discovery  that  no  two  stars  are  at  precisely  the 
same  stage  of  evolution. 

INTERIOR  OF  SUN 

Our  Sun  appears  to  be  still  gaseous  with  heat,  from  centre  to 
circumference.  But  its  constituent  gases  are  so  compressed  by 
the  attraction  of  its  huge  mass,  that  it  is,  on  the  average,  denser 
than  water.  It  has  in  fact  the  density  of  a  liquid  with  the 
mobility  of  a  gas.  In  other  words,  it  is  composed  of  glowing 
"gaseous-liquids,"  the  central  layers  of  which  are  extremely 
dense  (for  gases),  while  the  outer  layers  are  only  moderately  so. 

The  gaseous-liquid  nature  of  the  interior  is  shown  by  the 
average  density  of  the  Sun,  which  is  1.4  that  of  water.  This  is 
about  what  might  be  expected  for  a  huge  mass  of  intensely  hot 
gas,  but  is  far  too  small  for  solids  or  liquids,  taking  into  con- 
sideration the  fact  that  solar  gravitation  is  more  than  27  times 
as  great  as  that  of  the  Earth. 

That  the  interior  is  not  solid  may  also  be  seen  from  the  fact 
that  the  surface  heat  is  kept  up  by  a  continuous  supply  of  hot 
material  from  below.  This  interior  heat  is  actually  due  to  the 
contraction  or  shrinkage  of  the  outer  layers  of  the  Sun  as  they 


126     HOW  TO   KNOW  THE   STARRY   HEAVENS 

cool  off.  The  whole  Sun  may  be  said  to  boil,  the  hot  gases 
forcing  their  way  (explosively  or  otherwise)  to  the  surface,  cool- 
ing off  somewhat,  and  then  sinking.  If  the  interior  were  solid, 
or  if  a  solid  crust  were  to  form,  this  "  boiling  "  circulation  would 
cease,  the  heat  would  not  be  so  free  to  rise,  and  the  surface 
would  lose  its  heat  and  become  dark.  Some  day  this  will  take 
place,  and  we  shall  then  have  to  look  for  another  source  of  heat, 
or  get  used  to  the  intense  cold  of  interstellar  space. 

This  is  the  period  referred  to  by  George  Sterling  when  he 
says: 

"  The  Night  inevitable  waits 

Till  fails  the  insufficient  Sun, 
And  darkness  ends  the  toil  begun 
By  Chaos  and  the  morning  Fates. 

"  And  star  ward  drifts  the  stricken  world, 
Lone  in  unalterable  gloom, 
Dead,  with  a  Universe  for  tomb, 
Dark,  and  to  vaster  darkness  whirled." 

—  The  Testimony  of  the  Suns. 

Judging  from  the  Sun's  outer  characteristics,  it  is  almost  cer- 
tain that  there  are  no  compound  substances  in  its  interior.  It 
is  even  possible  that  the  so-called  elements  themselves  are  dis- 
sociated, by  the  intense  heat,  into  one  primitive  form  of  matter, 
which  exists  only  in  the  gaseous-liquid  state. 

THE  PHOTOSPHERE 

The  solar  photosphere  is  a  dazzlingly  incandescent  globular 
shell  surrounding  and  concealing  the  main  body  of  the  Sun.  It 
is  indeed  the  only  part  of  the  Sun  which  is  usually  visible  to 
us.  It  is  composed  of  closely  packed  clouds  of  intensely  hot 
metallic  vapours. 

These  photospheric  clouds  are  probably  long  and  pillar-like, 
floating  upright  in  the  metallic  atmosphere  of  permanent  gases 
which  surrounds  the  main  body  of  the  Sun.  Under  ordinary 
circumstances  only  the  bright  tops  of  these  radial  clouds  are 
visible  to  us.  As  the  spaces  between  them  are  comparatively 


SOLAR   ARCHITECTURE  127 

dark,  the  result  is  that  the  entire  surface  of  the  Sun  has  a 
granulated  or  mottled  appearance  when  seen  through  a  power- 
ful telescope.  It  looks,  indeed,  like  a  piece  of  grey  cloth  stretched 
over  a  hoop,  with  rice-grains  or  snowflakes  thickly  scattered 
over  it. 

The  particles  composing  these  upright  floating  clouds  probably 
rise  (in  a  gaseous  state)  from  the  inconceivably  hot  interior  of 
the  Sun,  and  gradually  cool  off'  through  expansion  and  outward 
radiation.  At  a  certain  elevation  the  diminished  pressure  and 
temperature  cause  them  to  condense  into  the  brilliant  vaporous 
clouds  whose  tops  form  the  visible  photosphere. 

As  the  rising,  cooling,  and  condensing  processes  go  on  all 
around  the  solar  nucleus,  there  is  probably  a  continuous  del- 
uge of  white-hot  metallic  "  rain  "  descending  Sunward  from  the 
chilled  summits  of  these  radial  luminous  clouds. 

It  is  possible  that  in  the  case  of  refractory  elements  like  car- 
bon, calcium,  etc.,  the  liquid  drops  "  freeze  "  and  descend  in  a 
"hailstorm"  of  genuine  diamonds  and  solid  metal  shot.  If 
this  is  so,  it  is  evident  that  the  falling  "  hail "  will  be  remelted 
and  vaporised  as  soon  as  it  reaches  a  sufficiently  hot  layer  of 
gases.  The  extraordinary  brilliancy  of  the  solar  photosphere 
may  be  largely  due  to  the  clash  of  these  diamonds  and  metal- 
lic shot,  as  they  fall  back  in  a  continuous  incandescent  hail- 
storm onto  the  ascending  gases  beneath  them. 

FACUL.E  AND   SUN-SPOTS 

The  intense  activity  of  internal  physical  action  is  sometimes 
indicated  by  the  appearance  of  certain  dark  blotches  on  the 
otherwise  fair  face  of  the  photosphere.  These  are  commonly 
known  as  Sun-spots,  and  are  usually  surrounded  by  brilliant 
eruptive  patches  of  piled-up  photospheric  clouds  or  faculce. 

The  Sun-spots  are  more  or  less  irregular  black  hollows  which 
occasionally  form  in  the  cloudy  photosphere  on  each  side  of  the 
solar  equator,  and  are  seen  to  drift  around  as  the  Sun  rotates 
on  its  axis  (see  Figure  24).  They  are  most  abundant  at  rather 
irregular  intervals  of  about  eleven  years.  At  the  beginning  of 


128     HOW   TO   KNOW  THE   STARRY   HEAVENS 

one  of  the  Sun-spot  periods  the  spots  are  some  distance  north 
and  south  of  the  Equator.  Later  on,  the  Sun-spot  areas  move 
nearer  to  it,  so  that  the  last  spots  seen,  in  one  such  period,  are 
not  far  from  the  Equator. 

The  Sun-spots  usually  break  out  in  the  midst  of  a  piled-up 
mass  of  brilliant  faculce.  At  first  they  appear  as  irregular  black 
openings  in  the  bright  photosphere.  They  gradually  increase  in 
size  and  become  more  circular  in  outline.  The  lower  ends  of 
the  long  perpendicular  clouds  which  (presumably)  compose  the 
surrounding  photosphere  gradually  drift  into  the  opening,  so  that 
the  spot  becomes  a  saucer-shaped  depression.  The  black  bottom 
of  this  saucer  is  known  as  the  Sun-spot  nucleus,  and  the  semi- 
dark  fringed  sides  form  what  is  known  as  the  penumbra. 

The  nucleus  itself  is  sometimes  40,000  to  50,000  miles  in 
diameter,  the  surrounding  penumbra  being  occasionally  150,000 
miles  across.1 

Sun-spots  vary  in  depth  from  500  to  2,000  miles,  and  are 
filled  with  cooler  and  therefore  less  luminous  gases,  which  ab- 
sorb the  light  from  below.  The  blackness  is  only  the  result  of 
contrast,  for  their  gaseous  contents  are  really  hotter  and  brighter 
than  molten  steel.  There  appears  to  be  a  spiral  uprush  of  hot 
gas  all  around  the  spot,  with  a  descending  current  in  the  mid- 
dle. They  are  evidently  related  to  the  cyclones,  tornadoes, 
etc.,  in  the  temperate  regions  of  our  Earth.  The  surrounding 
penumbra  consists  of  jets,  swirls,  and  cataracts  of  luminous 
vapour  surging  into  the  abyss.  Long  plume-like  "  bridges  "  of 
superheated  gas  occasionally  shoot  out  from  the  sides  and  cross 
the  cavity,  which  is  finally  covered  up  by  dazzling  masses  of  facu- 
Ise  (see  Figures  25  and  69).  When  these  brilliant  inrushes  of 
vapour  are  crossing  the  abyss,  they  produce  an  electrical  activ- 
ity which  extends  far  out  into  space.  On  our  Earth  it  produces 
the  polar  phenomena  known  as  the  Aurora,  and  causes  power- 
ful currents  in  all  kinds  of  electrical  apparatus.  These  latter  are 
commonly  known  as  electrical  storms. 

1  If  the  great  Sun-spot  of  1858  had  been  35  times  larger,  it  would  have  cov- 
ered the  entire  surface  of  the  Sun,  and  practically  put  it  out  of  business  for  the 
time,  so  far  as  illuminating  and  heating  purposes  are  concerned. 


SOLAR   ARCHITECTURE  129 


ROTATION 

An  examination  of  the  time  which  it  takes  the  spots  and 
granulations  of  the  photosphere  to  move  across  the  disc  as 
the  Sun  rotates  shows  that  the  clouds  at  a  distance  from  the 
Equator  take  a  longer  time  to  complete  a  rotation  than  those 
on  or  near  to  the  Equator.  While  the  Equatorial  granulations 
rotate  in  25J  days,  the  spots  take  more  than  a  day  longer  to 
complete  their  rotation.  And  the  granulations  near  the  Poles 
take  about  40  days  to  go  once  around.  This  shows  the  gaseous 
mobility  of  a  great  part,  and  probably  of  the  entire  mass,  of  the 
Sun. 

THE  CHROMOSPHERE 

I  have  said  that  the  dazzling  white  clouds  which  form  the 
visible  photosphere  float  in  an  atmosphere  of  uncondensed  gases. 
This  metallic  atmosphere,  or  veil,  is  ruddy,  but  tolerably  trans- 
parent. It  extends  4,000  or  5,000  miles  above  the  luminous 
clouds,  and  has  a  ragged  storm-tossed  surface  (see  Figure  27). 
It  is  known  as  the  chromosphere,  sierra,  or  solar  atmosphere, 
The  lower  portion  of  it  (about  500  miles  thick)  is  known  as 
the  "  reversing  layer."  a  The  spectroscope  has  shown  that  this 
lower  and  denser  portion  consists  of  metallic  gases,- with  some 
non-metallic  elements,  all  free  and  uncombined  on  account  of 
the  intense  heat.  The  most  abundant  (or  at  least  the  most 
recognisable),  are  hydrogen,  calcium,  iron,  manganese,  nickel, 
and  titanium.  The  following  are  also  present :  barium,  carbon, 
chromium,  cobalt,  germanium,  helium,  magnesium,  platinum, 
silicon,  silver,  sodium,  and  zinc.  There  is  also  strong  evidence 
of  the  presence  of  aluminium,  cadmium,  copper,  lead,  molybde- 
num, oxygen,  palladium,  uranium,  and  vanadium. 

In  addition  to  these  elements,  there  are  lines  indicating  the 
existence  of  substances  as  yet  unknown  on  Earth. 

1  This  name  is  derived  from  the  fact  that  a  spectrum  formed  from  its  light  alone 
has  the  usual  dark  solar  absorption  lines  reversed  into  bright  radiation  lines.  This 
' '  flash  spectrum  "  can  be  obtained  for  only  a  few  seconds  at  the  beginning  or  end 
of  a  solar  eclipse. 

9 


130     HOW  TO   KNOW  THE   STARRY   HEAVENS 

There  are  no  signs  of  chlorine,  nitrogen,  gold,  mercury,  phos- 
phorus, sulphur,  and  some  other  elements.  This  does  not, 
however,  prove  that  they  are  not  present  in  other  parts  of  the 
Sun. 

The  upper  and  thinner  portion  of  the  sierra  does  not  contain 
the  numerous  metals  found  in  its  lower  "  reversing  layer."  It 
is  composed  of  hydrogen,  helium,  and  one  or  two  other  perma- 
nent gases.1 

ERUPTIVE  PROMINENCES   (METALLIC  FLAMES) 

In  the  Sun-spot  zones  on  either  side  of  the  Equator,  immense 
red  jets  of  glowing  gas  rise  between  the  white  clouds  of  the 
photosphere,  pass  through  the  sierric  atmosphere,  and  extend 
many  thousands  of  miles  above  it.  They  are  known  as  eruptive 
prominences  or  metallic  flames  (see  Frontispiece  and  Figures  26, 
27,  and  70).  The  highest  one  ever  seen  rose  more  than 
350,000  miles  above  the  general  surface. 

These  red  metallic  flames  are  best  seen  when  on  the  edge  of 
the  Sun.  When  they  are  on  our  side  they  are  invisible  with 
the  telescope,  but  their  upper  surfaces  can  be  photographed  (at 
different  elevations)  with  the  aid  of  Professor  Hale's  spectro- 
heliograph.  In  this  position  they  are  known  as  Eruptive  Cal- 
cium Flocculi  (see  Figure  128). 

These  eruptive  flames,  or  flocculi,  consist  chiefly  of  gaseous 
calcium,  hydrogen,  sodium,  magnesium,  and  iron.  They  ap- 
pear to  be  forced  up  by  explosive  physical  changes  below  the 
photosphere,  but  their  great  elevation  is  probably  due  to  the  re- 
pulsive action  of  light  on  the  small  particles  of  which  they 
consist. 

CLOUD  PROMINENCES  (HYDROGEN  FLAMES) 

Above  the  sierra  or  chromosphere  there  are  also  to  be  seen  huge 
ruddy  clouds  consisting  of  hydrogen  and  helium.  When  they 
are  seen  in  profile,  at  the  edge  of  the  Sun,  they  are  known  as 

1  The  non-metallic  elements  are  here  printed  in  italics. 


SOLAR   ARCHITECTURE  131 

cloud  prominences  or  hydrogen  flames.  When  they  are  on 
our  side  of  the  Sun,  and  are  photographed  from  overhead 
by  means  of  the  spectroheliograph,  they  are  called  hydrogen 
flocculi.1  They  are  not  confined  to  latitudinal  belts  or  zones, 
like  the  eruptive  ones  just  mentioned,  but  are  to  be  found  over 
all  parts  of  the  Sun's  surface.  There  does  not  seem  to  be  any 
atmosphere  for  them  to  float  in.  Like  the  eruptive  prom- 
inences, they  appear  to  be  composed  of  extremely  small  particles 
of  matter,  upborne  by  the  repulsive  action  of  the  Sun's  light. 


FIG.  71.  — A  SOLAR  "CLOUD"  OF  GLOWING  HYDROGEN 
(PROFESSOR  YOUNG) 

About  100,000  miles  long  and  54,000  miles  high.     Notice  the 
bright  uprush  below  one  end  of  it. 

In  1871  Professor  Young  observed  a  hydrogen  cloud  about 
100,000  miles  long,  with  its  summit  about  54,000  miles  above 
the  sierra.  It  was  connected  with  this  latter  by  a  number  of 
vertical  columns  that  looked  like  water-spouts  (see  Figure  71). 
Thirty-five  minutes  later  the  cloud  had  been  blown  to  pieces 
by  an  eruption,  and  the  fragments  were  scattered  to  a  height  of 
207,000  miles  (see  Figure  72).  They  gradually  faded  away  as 
they  rose,  leaving  an  eruptive  prominence  50,000  miles  in  height 
(see  Figure  73).  The  flame-like  summit  of  this  eruptive  mass 
rolled  over  like  a  breaking  wave  (see  Figure  74),  and  in  a  few 
minutes  faded  out  of  sight.  These  changes  all  took  place  in 
about  two  hours. 

1  For  further  particulars  concerning  the  spectroheliograph  and  the  latest 
results  obtained  with  its  aid,  see  an  illustrated  article  by  Professor  Hale  in  the 
"Popular  Science  Monthly,"  for  May,  1904. 


132     HOW  TO   KNOW  THE   STARRY   HEAVENS 


THE  CORONA 

The  white  spherical  cloud-surface  of  the  Sun  has  now  been 
described,  and  also  the  ruddy  atmosphere  and  the  two  kinds  of 

prominences  which  rise 
from  it.  But  outside  of 
all  these  there  is  a  ra- 
diating halo  of  pearly 
light  sometimes  ex- 
tending more  than 
2,000,000  miles  in  every 
direction  (see  Figures 
26,  28,  and  75).  This 
is  known  as  the  corona. 
Like  the  eruptive  prom- 
inences, it  appears  to 
start  from  volcanic 
outbursts;  but,  its 
particles  being  smaller, 
instead  of  being  merely 
upheld  by  the  radiant 
energy  of  the  Sun,  they 
are  violently  repelled, 
so  that  they  stream 
forth  continuously  and 
pass  away  into  outer 
space.  When  Sun- 
spots  are  numerous,  the 
visible  corona  does  not 
extend  so  far  as  when  they  are  absent  or  scarce.  The  greatest 
development  of  the  corona  is  over  the  Sun-spot  zones  north  and 
south  of  the  Equator.  The  rotation  of  the  Sun  causes  the 
coronal  streamers  to  bend  in  a  plume-like  manner.  They  are 
not  so  crowded  together  near  the  poles  of  the  Sun,  so  that 
the  polar  streamers  are  very  distinct  (see  Figure  29). 

When  the  spectroscope  is  turned  to  this  coronal  halo  it  shows 
a  continuous  spectrum  crossed   by  bright  lines.     The   former 


FIG.  72.  —  THE  SAME  REGION  35  MINUTES  LATER 

(YOUNG) 

Rising  wisps  of  glowing  hydrogen,  reaching  a  height 
of  207,000  miles.  The  eruptive  mass  below  is  growing 
larger. 


FIG.   70.  —  SOLAK   "FLAMES"  OR  PROMINENCES 

By  Zoellner.     (From  Comstock's  "  Text-book  of  Astronomy,"  published  by 
Messrs.  D.  Appleton  &  Co. ) 


FIG.   76.  —  THEORETICAL  SECTION  OF  SOLAR  PHOTOSPHERE 
By  Trouvelot. 

Clouds  of  carbon,  etc.,  floating  in  an  atmosphere  of  metallic  gases.     Hot  gases  from 

interior  forcing  their  way  out.    (From  Todd's  "  New  Astronomy," 

published  by  The  American  Book  Co.) 


SOLAR  ARCHITECTURE 


133 


FIG.  73.  — THE  SAME    REGION    35 
MINUTES  LATER  (YOUNG) 

The  uprush  has  developed  into  a  mass 
of  rolling  and  ever-changing  "flame," 
50,000  miles  in  height. 


indicates  the  presence  of  solid  or  liquid  particles,  and  the 
bright  lines  are  those  of  glowing  hydrogen  gas.  There  is  also 
a  bright  green  line  due  to  an 
unknown  gas  which  has  been 
provisionally  named  "coro- 
nium."  The  atoms  of  these  sub- 
stances are  so  far  apart  that  the 
corona  is  practically  a  vacuum. 

SOLAR  ENERGY 

As  in  the  lime-light,  or  oxy- 

calcium  lamp  (used  for  magic 

lanterns,  etc.),  the  brilliancy  of 

the  Sun's  photosphere  is  largely 

due    to   incandescent    calcium. 

The  intense  bluish-white  light 

of    this   hollow   cloudy   sphere 

forces  its  way  through  the  ruddy  transparent  atmosphere  and 

prominences.     In  doing  so  it  loses  much  of  its  intensity  and 

becomes  slightly  yellow. 
It  passes  through  the 
coronal  streamers  with 
but  little  loss,  and 
spreads  out  evenly  in 
all  directions. 

The  planets  catch  a 
little  of  this  scattered  ra- 
diance, but  the  bulk  of 
it  is  lost  in  the  outer 
darkness  of  space.  If 
the  portion  of  light  and 
heat  received  by  the 
Earth  be  represented  by 
the  figure  1,  the  part 

which  misses  the  Earth  will  be  represented  by  2,200,000,000. 

So  that  what  we  get  is  equal  to  one  cent  out   of  twenty-two 


FIG.  74.  _  THE  SAME,  15  MINUTES  LATER 
(YOUNG) 

In  half  an  hour  this  curling  prominence  faded  entirely 
away. 


134     HOW  TO   KNOW  THE   STARRY   HEAVENS 

millions  of  dollars.  What  all  the  planets  catch  is  equal  to  ten 
cents  out  of  the  same  amount  of  money.  All  the  rest  escapes 
into  outer  space  and  is  apparently  wasted. 

The  amount  of  "  waste  "  may  be  conceived  from  the  fact  that 
(in  spite  of  the  loss  by  absorption  in  passing  through  the  sierra, 
etc.)  every  square  foot  of  the  Sun's  surface  continually  sends 
out  into  space  about  10,000  horse-power  of  radiant  energy, 
while  a  square  foot  of  our  Earth's  surface  receives  about  a 
quarter  of  one  horse-power.1 

CAUSE  OF  SOLAR  HEAT 

Although  we  cannot  see  what  is  taking  place  beneath  the 
photosphere  of  the  Sun,  we  can  learn  something  of  its  anatomy 
and  physiology  by  indirect  methods.  All  the  surface  phenom- 
ena, revealed  by  the  telescope  and  spectroscope,  are  due  to 
deep-seated  mechanical  and  physical  processes.  Our  knowledge 
of  the  effects  of  known  mechanical  and  physical  laws  enables 
us  to  form  some  idea  of  what  is  going  on  below.  The  whole 
body  of  the  Sun  appears  to  be  composed  of  concentric  gaseous 
layers,  like  the  coats  of  an  onion.  Each  shell  is  denser  than 
the  layer  above  it,  and  therefore  thinner  than  the  one  below. 
As  the  outer  layers  cool  off  by  the  radiation  of  their  heat  into 
space,  they  settle  down  onto  the  lower  layers  and  urge  them  to 
a  quicker  rotation.  This  makes  the  Sun  a  huge  electrical 
machine,  generating  an  enormous  amount  of  energy,  which 
manifests  itself  in  different  forms.  When  the  various  layers 
have  reached  a  certain  density,  there  is  a  periodical  overheating 
of  the  lower  layers  through  the  progressive  cooling  of  the  upper 
ones.  This  periodical  overheating  causes  the  layers  to  react 
on  one  another  with  great  violence.  The  overheated  gases 
escape  at  every  weak  place,  and  these  periodical  outbursts  of 
energy  are  visible  on  the  surface  in  the  form  of  eruptive  promi- 
nences, Sun-spots,  etc.  (see  Figure  76). 

1  This  latter  would  in  one  year  lift  GO  short  tons  one  mile  high ;  the  former 
would  lift  40,000  times  as  much. 


SOLAR   ARCHITECTURE  135 

So  much  for  the  physical  constitution  of  the  Sun.  If  the 
reader  will  now  turn  back  to  the  section  entitled  "  Classes  of 
Stars,"  in  Chapter  VIII,  he  will  be  able  to  form  some  idea  as  to 
the  physical  condition  of  nebulae,  stars,  and  planets,  at  different 
stages  of  evolution. 

OLD  AND  DYING  SUNS 

The  common  variability  of  red  stars  is  evidently  the  outer 
manifestation  of  the  death-struggles  of  old  and  waning  suns. 
The  vital  forces  of  the  aged  stars  are  no  longer  able  to  prevent 
the  various  elements  from  forming  chemical  combinations, 
resulting  in  periodic  fluctuations.  Their  interior  forces  are 
making  tremendous  but  unsuccessful  efforts  to  throw  off  the 
cooling  vapours  above,  which  are  gradually  choking  the  life  out 
of  the  dying  suns.  Some  of  them  are  nearly  invisible,  except 
when  these  paroxysms  are  at  their  height.  During  these  peri- 
odical spasms,  they  glare  out  strangely  into  the  darkness  for  a 
time,  and  then  subside.  Their  struggles  grow  feebler  as  time 
goes  on,  and  finally  their  uneasy  flickering  subsides  into  the 
calmness  and  tranquillity  of  solar  death. 


CHAPTER  XII 

A  REELING  WORLD 

"  Learned  Faustus, 
To  know  the  secrets  of  astronomy, 
Graven  in  the  book  of  Jove's  high  firmament, 
Did  mount  himself  to  scale  Olympus'  top, 
Being  seated  in  a  chariot  burning  bright, 
Drawn  by  the  strength  of  yoked  dragons'  necks. 

When  Faustus  had  with  pleasure  ta'en  the  view 
Of  rarest  things,  and  royal  courts  of  kings, 
He  stayed  his  course  and  so  returned  home  ; 
Where  such  as  bare  his  absence  but  with  grief, 
I  mean  his  friends  and  near'st  companions, 
Did  gratulate  his  safety  with  kind  words, 
And  in  their  conference  of  what  befel, 
Touching  his  journey  through  the  world  and  air, 
They  put  forth  questions  of  astrology, 
Which  Faustus  answer'd  with  such  learned  skill 
As  they  admir'd  and  wonder'd  at  his  wit." 

—  Marlowe,  ' '  Faustus. " 

A  BIRD'S-EYE  VIEW 

IN  order  to  ascertain  some  more  particulars  about  our  own 
Earth,  let  us  go  back,  for  a  time,  to  our  Chariot  of  Imagi- 
nation, and  once  more  watch  our  Solar  System  from  a  short 
distance  away. 

Let  us  suppose  that  we  are  so  situated  in  space  that  the  Sun 
and  planets  appear  as  though  they  were  floating  in  front  of  us, 
on  the  surface  of  a  sheet  of  perfectly  clear  water.  And  let  us 
suppose  that  (owing  to  a  slight  eddy  in  the  water)  the  planets 
are  all  circling  slowly  around  the  Sun.  When  they  are  between 
us  and  the  Sun,  they  drift  to  the  right,  but  when  they  are  beyond 
it  they  go  to  the  left.  The  surface  of  this  imaginary  sheet  of 


A   REELING  WORLD 


137 


water  will,  to  an  inhabitant  of  the  Third  Planet,  represent  the 
plane  of  the  Ecliptic,  the  so-called  path  of  the  Sun. 

From  our  supposed  position  in  space,  we  can  not  only  see  our 
own  Solar  System  spread  out  before  us,  but  also,  in  the  far  dis- 
tance, the  innumerable  starry  systems  which  surround  it.  Those 


Pisces. 
FIG.  77.  —  DIAGRAM  ILLUSTRATING  ZODIAC 


stars  which  are  on  or  near  the  horizon  of  our  imaginary  sheet 
of  water  may  be  conveniently  divided  into  twelve  equal  groups 
or  constellations.1 

Among  these  twelve  Signs  of  the  Zodiac,  or  Ecliptic,  are 
those  which  bear  the  names  of  Sagittarius,  Virgo,  Gemini,  and 
Pisces.  As  these  constellations  will  hereafter  be  used  to  indi- 


1  Named  as  follows : 

1.  Aries,  the  Ram.         5.  Leo,  the  Lion.  9.  Sagittarius,  the  Archer. 

2.  Taurus,  the  Bull.       6.  Virgo,  the  Virgin.  10.  Capra,  the  Goat. 

3.  Gemini,  the  Twins.    7.  Libra,  the  Balance.  11.  Aquarius,  the  Water-Bearer. 

4.  Cancer,  the  Crab.      8.  Scorpio,  the  Scorpion.  12.  Pisces,  the  Fishes. 


138     HOW  TO   KNOW  THE   STARRY   HEAVENS 

cate  planetary  motions,  their  relative  positions  on  the  Ecliptic 
are  here  indicated  (see  Figure  77). 

Keeping  these  positions  in  mind,  let  us  now  turn  our  eyes 
back  to  our  own  Solar  System. 

If  we  get  out  our  telescopes  and  watch  the  Third  Planet,  which 
is  now  known  to  us  as  the  Earth,  we  shall  see  that  its  North 
Pole  is  uppermost,  but  that  it  has  a  heavy  list  to  the  right.  As 
the  globe  spins  around,  the  markings  on  its  near  side  move  to 
the  right,  with  a  slight  downward  tendency  due  to  the  planet's 
inclination. 

One  result  of  the  planet  not  floating  quite  erect  is  that  the 
rotation  causes  its  tropical  regions  to  be  continually  dipping 
into  the  water  and  emerging  from  it,  while  the  rest  of  the  globe 
remains  all  the  time  either  above  or  below  the  surface,  which 
represents  the  plane  of  the  Ecliptic. 

On  turning  to  the  other  planets  for  comparison,  we  find  that 
they  are  rotating  in  the  same  direction.  Some  of  them  have 
satellites,  or  moons,  and  these  appear  to  drift  around  their 
primaries  in  the  same  direction,  as  though  the  rotation  of  each 
planet  caused  a  secondary  eddy  in  the  water.  It  will  be  seen, 
therefore,  that  all  these  movements  —  (1)  the  revolution  of  the 
planets  around  the  Sun,  (2)  their  rotation  around  their  axes,  and 
(3)  the  revolutions  of  the  satellites  around  their  primaries  —  are 
in  the  opposite  direction  from  that  taken  by  the  hands  of  a 
watch.1 

A  closer  view  of  the  satellites  would  show  that  their  rotation 
also  agrees  in  direction  with  the  three  motions  just  described. 
Some  of  the  satellites  have  adjusted  the  speed  of  their  rotation 
to  that  of  their  revolution,  so  that  they  always  show  the  same 
side  to  the  world  they  attend.2 

It  is  hardly  necessary  to  say  that  all  these  agreements  are  not 

1  The  satellites  of  the  two  outside  planets  do  not  at  present  conform  to  the 
general  rule,  though  they  probably  will  in  the  course  of  time.     These  planets  ap- 
pear to  have  been  formed  later  than  any  of  the  others. 

2  The  two  inner  planets  appear  to  have  done  the  same  thing,  so  that  they 
always  turn  the  same  side  to  the  Sun. 


A    REELING   WORLD  139 

accidental.  There  is  a  cause  for  them,  as  will  be  seen  in  a  later 
chapter. 

If  we  stay  where  we  are,  and  watch  the  movements  of  the 
Earth  for  a  few  years,  we  shall  see  that  while  it  drifts  around 
the  Sun  its  axis  keeps  the  same  position.  The  result  is  that  a 
person  living  at  the  North  Pole  would  have  the  same  star  over- 
head all  the  year  round. 

When  the  Earth  is  to  the  right  of  the  Sun,  it  is  obvious  that 
the  North  Pole  is  in  the  dark,  and  that  it  is  winter  in  the  North- 
ern Hemisphere  (see  Figure  12).  When  the  Earth  gets  beyond 
the  Sun,  both  poles  are  just  lighted  up  by  it,  and  the  days  and 
nights  are  equal  all  over  the  globe.  This  is  known  in  the  North- 
ern Hemisphere  as  the  Spring  Equinox.  When  it  arrives  at 
the  left  of  the  Sun,  the  North  is  enjoying  its  summer,  and  the 
South  Pole  is  in  the  dark.  Finally,  when  the  Earth  reaches 
our  side  of  the  Sun,  the  days  and  nights  are  again  equal  all 
over  the  globe.  This  is  known  in  the  North  as  the  Autumnal 
Equinox.  As  already  stated,  the  axis  of  the  Earth  does  not 
share  in  any  of  these  movements  of  the  planet,  but  remains  fixed 
with  regard  to  the  stars. 

Let  us  now  see  what  ideas  the  inhabitants  of  this  Third  Planet 
are  likely  to  have  with  regard  to  the  various  movements  just 
described.  The  rotation  of  the  planet  itself  they  will  not  be 
able  to  perceive,  so  that,  as  it  spins  toward  the  east,  they  will 
naturally  fall  into  the  delusion  that  the  stars  themselves  are  roll- 
ing over  toward  the  west.  They  will  also  naturally  think  that 
the  Sun  and  planets  are  doing  the  same  thing.  But  as  they  are 
also  ignorant  of  the  fact  that  their  Earth  is  leaning  over  to  one 
side,  they  will  be  surprised  and  puzzled  to  find  that  the  Sun 
gradually  drifts  toward  the  north  and  south  as  summer  and 
winter  approach.  They  will  also  find  it  hard  to  account  for  the 
erratic  manner  in  which  the  planets  appear  to  move,  or  for  the 
fact  that  they  always  keep  on  or  near  the  Sun's  path. 

To  an  outside  observer  these  circumstances  seem  to  be  too 
simple  to  require  explanation,  but  to  an  inhabitant  who  can 
neither  see,  hear,  nor  feel  that  his  own  world  is  moving,  the 


140    HOW  TO   KNOW  THE   STARRY   HEAVENS 

whole  of  the  celestial  motions  are  puzzling  to  the  last  degree. 
It  speaks  well  for  the  intelligence  of  the  inhabitants  that  many 
of  them  have  at  last  managed  to  find  out  the  real  facts  of  the 
case. 

As  the  Earth  rolls  over  every  day  there  are  two  points  on  the 
surrounding  "  star-sphere  "  which,  to  an  inhabitant  of  the  planet, 
do  not  appear  to  move,  but  seem  to  be  centres  of  rotation.  These 
two  points  are  opposite  the  North  and  South  Poles  of  rotation. 
If  we  let  the  eye  follow  the  direction  of  the  axis,  in  an  upward 
(but  slanting)  direction,  till  it  reaches  the  stars,  it  will  be  found 
that  there  happens  to  be  a  tolerably  bright  star  near  the  point 
around  which  the  other  stars  appear  to  move.  To  an  inhab- 
itant of  the  Earth  this  star  therefore  becomes  known  as  the 
North  Pole-Star.  Let  us  keep  this  in  mind,  for  if  the  axis  of 
the  Earth  never  changes  its  position  this  star  will  always  re- 
main the  North  Pole-Star  so  far  as  the  Earth  is  concerned.1 

There  is  another  peculiarity  about  the  Earth's  motions  that  is 
worth  mentioning.  The  World,  like  all  the  other  planets,  does 
not  go  around  the  Sun  in  an  exact  circle,  but  in  a  nearly  circular 
ellipse  or  oval.  The  result  of  this  is  that  at  one  part  of  the 
year  the  Earth  is  about  3,000,000  miles  nearer  to  the  Sun  than 
it  is  six  months  earlier  or  later. 

From  our  chosen  point  of  observation  the  Earth  is  to  the 
right  of  the  Sun  on  the  21st  day  of  December  in  each  year,  and 
its  North  Pole  is  then  turned  exactly  away  from  the  Sun.  This 
part  of  the  Earth's  orbit  is  termed  the  Winter  Solstice,  and  we 
will  call  it  the  point  A.  From  the  Earth,  the  Sun  then 
appears  to  be  in  the  sign  or  constellation  of  Sagittarius  (see 
Figure  77). 

A  few  days  later  the  Earth  reaches  that  part  of  its  orbit  where 
it  is  nearest  to  the  Sun.  This  part  of  the  orbit  is  termed  its 
Perihelion,  and  we  will  call  it  the  point  B. 

1  In  "Julius  Caesar,"  Shakespeare  makes  one  of  his  characters  say: 

"  I  am  constant  as  the  Northern  Star, 
Of  whose  true-fixed  and  resting  quality 
There  is  no  fellow  in  the  Firmament." 


A   REELING   WORLD  141 

OUR  FIRST  VISIT  (A.D.  1900) 

Let  us  note  the  relative  position  of  these  two  points  in  the 
year  1900  A.D.  (10°  apart,  measured  from  the  Sun).  Then  we 
will  wait  for  a  few  thousand  years,  so  as  to  see  whether  the  two 
phenomena  always  continue  to  happen  in  the  same  places. 

SECOND  VISIT   (A.D.   8400) 

After  amusing  ourselves  by  travelling  among  the  stars  for 
6,500  sidereal  years  we  return  to  our  System  (in  the  year  8400 
A.  D.),  and  look  for  the  Third  Planet.  It  is  still  spinning  away 
like  a  top  that  is  wound  up  for  ever.  But,  strange  to  say,  there 
has  been  a  remarkable  change  in  the  position  of  the  two  points 
A  and  B.  The  northern  end  of  the  polar  axis,  instead  of  point- 
ing away  from  the  Sun  when  the  Earth  was  to  our  right  (see 
Figure  12),  has  reeled  slowly  back  (or  retrograded)  through  a 
quarter  circle.  Its  opposition  (A)  to  the  Sun,  therefore,  takes 
place  when  the  Earth  is  between  us  and  the  Sun.  If  we  regard 
the  Earth  as  the  hand  of  a  watch  (going  around  the  Sun  once  a 
year,  the  opposite  way  to  the  hands  of  our  watches),  it  is  evi- 
dent that  this  watch  has  gained  a  quarter  of  a  year  (90°  of  arc, 
measured  from  the  Sun).  For  the  inhabitants  will  tell  you  that 
it  is  the  21st  day  of  December,  while  it  is  evident  that  it  ought 
to  be  the  22d  of  September  so  far  as  the  earthly  revolutions  are 
concerned. 

The  result  of  this  is  that  the  Earth  has  now  another  Pole-Star, 
and  the  old  one  appears  (to  the  creatures  on  the  Earth)  to  go 
circling  around  it  like  all  the  other  stars.  The  twelve  monthly 
"  mansions  of  the  Sun  "  have  also  swung  a  quarter  round,  so  that 
they  are  occupied  by  different  sets  of  stars.  From  the  Earth, 
the  December  Sun  now  appears  to  be  in  the  constellation  of 
Virgo  instead  of  being  in  Sagittarius  as  before  (see  Figure  77). 

Let  us  see  whether  there  has  been  any  change  in  the  peri- 
helion (B),  the  place  where  the  Earth  is  nearest  to  the  Sun.  Yes, 
it  has  moved  forward  in  the  orbit  about  20°  of  arc.  The  two 
points  A  and  B,  which  were  only  10°  apart,  are  now  120°  apart 


142     HOW  TO   KNOW  THE   STARRY   HEAVENS 

(100+90°+20°=1200).  The  combined  result  of  the  two  changes 
is  that,  instead  of  being  nearest  to  the  Sun  at  the  beginning  of 
January,  the  Earth  is  now  in  perihelion  toward  the  end  of 
April. 

THIRD  VISIT  (A.D.   14900) 

Let  us  now  go  away  a  second  time  for  6,500  years,  and  then 
come  back  to  our  Solar  System  again  (in  the  year  14900  A.  D.). 

The  points  A  and  B  have  gone  on  separating  at  the  same 
rate,  so  that  the  angle  between  them  is  now  230°  of  arc  (10°+ 
1100+110°  =  2300),  measured,  as  before,  from  the  Sun.  It  is 
therefore  December  21  when  the  Earth  is  to  the,  left  of  the  Sun. 
A  third  and  distant  Pole-star  has  replaced  the  second  one.  The 
December  Sun  is  now  in  the  constellation  of  Gemini,  and  the 
Earth  is  nearest  to  the  Sun  in  August. 

FOURTH  VISIT   (A.D.   21400) 

A  third  time  we  retire  for  the  same  length  of  time,  and  then 
go  back  to  our  old  acquaintance  (A.  D."  21400).  By  this  time 
the  Earth-clock  has  gained  three  quarters  of  a  year  with  refer- 
ence to  the  stars,  or  nearly  a  year  with  reference  to  its  peri- 
helion (10°+1100+1100+110°  =  340°).  The  Earth  is  now  beyond 
the  Sun  in  December.  A  fourth  Pole-star  has  replaced  the 
third  one.  The  December  Sun  is  now  in  the  constellation  of 
Pisces,  and  the  Earth  is  again  nearest  to  the  Sun  in  December. 

FIFTH  VISIT  (A.D.   27900) 

Once  more  we  go  away  for  the  same  length  of  time.  On  our 
return  (in  the  year  A.D.  27900),  the  points  A  and  B  have  passed 
each  other.  The  Earth-clock  has  gained  a  whole  year  with 
reference  to  the  stars,  so  that  the  inhabitants  tell  us  we  are  a 
year  overdue.  December  21  has  at  last  reached  its  old  camping- 
ground  to  the  right  of  the  Sun  (as  in  Figure  12).  The  old  Pole- 
star,  that  we  knew  so  well  26,000  years  ago,  has  returned  to  its 
place  of  duty.  And  so  have  the  twelve  Signs  of  the  Zodiac,  so 
that  the  December  Sun  is  once  more  in  the  constellation  of 


A   REELING   WORLD 


143 


Sagittarius.  But  the  points  A  and  B  are  now  90°  apart,  so  that 
the  Earth,  instead  of  being  nearest  to  the  Sun  at  the  opening  of 
the  year  (as  at  our  first  visit,  in  the  year  A.  D.  1900),  does  not 
reach  its  perihelion  until  March.  Figure  78  shows  the  posi- 
tions of  A  and  B  at  the  various  dates  mentioned. 


FIG.  78.  —  DIAGRAM  ILLUSTRATING  PRECESSION  OP  EQUINOXES  AND 
ADVANCE  OF  PERIHELION 

The  places  marked  A  1,  2,  3,  4,  5,  are  the  positions  of  the  Earth  at  the  Winter 
Solstice,  at  the  dates  mentioned.  The  places  marked  B  1,  2,  3,  4,  5,  are  the  posi- 
tions where  the  Earth  is  nearest  to  the  Sun  at  the  dates  mentioned. 


We  have  now  followed  the  backward  reeling  of  the  Earth's 
axis  for  one  complete  re  volution,  which  has  taken  26,000  years.1 
This  reeling  of  the  Earth's  poles  around  the  poles  of  the  Ecliptic 
is  known  as  the  Precession  of  the  Equinoxes,  because  it  makes 

1  More  exactly,  25,868  years. 


144    HOW  TO   KNOW  THE   STARRY   HEAVENS 

the  spring  and  autumn  equinoxes  come  a  little  sooner  every 
revolution  than  they  would  otherwise  do.  It  is  in  some  respects 
similar  to  the  slow  wabbling  motion  of  a  child's  top.  We  may 
almost  say  that  our  World  is  a  big  top  which  spins  and  wabbles 
as  it  swings  around  the  Sun.1 

EIGHTEENTH  VISIT 

If  we  were  to  keep  up  our  periodical  visits  to  the  Earth  till 
the  point  B,  where  it  is  nearest  to  the  Sun,  has  advanced  one 
complete  revolution  to  its  old  place,  it  would  take  about  13  more 
visits,  6,500  years  apart.  For  this  point,  called  the  Earth's 
perihelion,  takes  109,000  years  to  complete  its  circuit. 

MILLIONS  OF  YEARS  LATER 

Suppose  that  we  now  leave  our  System  for  —  say  x  millions 
of  years  —  and  then  come  back  to  it.  What  changes  shall  we 
find  on  our  return  ? 

In  the  first  place  we  shall  have  some  trouble  in  finding  our 
Solar  System  at  all,  for  in  the  meantime  it  has  drifted  so  far 
that,  if  its  curved  path  were  straightened  out,  it  would  be  equal 
in  length  to  Sx  times  the  distance  of  Sirius.  And  its  Sun  has 
changed  from  a  yellowish-white  star  to  a  dark-red  one,  with 
periodical  spasms  of  brilliancy. 

In  the  second  place,  when  we  have  found  it,  picked  out  the 
wizened  relic  of  our  once-beautiful  Earth,  and  taken  our  star- 
photographs,  we  shall  discover  that  the  almost  inperceptible 
drifting  of  the  stars  has  by  this  time  completely  changed  the 
face  of  the  heavens.  The  "  sweet  influences  "  of  the  Pleiades 
(Job  xxxviii,  31)  have  now  been  dissipated  and  lost.  The 
"  Bands  of  Orion  "  are  loosed  for  ever.  The  "  Mazzaroth  "  (or 
Signs  of  the  Zodiac)  are  no  more  waiting  to  be  led  forth  in 
their  season.  And  the  Great  Bear,  with  her  cub,  no  longer  re- 
mains to  be  led  around  the  "  pole."  Of  all  the  constellations 

1  The  cause  of  the  Precession  of  the  Equinoxes  will  be  dealt  with  in  chapter 
XV. 


A   REELING  WORLD^  145 

we  used  to  know  so  well,  not  one  is  now  recognisable,  though 
myriads  of  shining  suns  still  sparkle  in  the  ebon  "  vault." 

On  this  our  last  visit  to  the  Solar  System  we  not  only  find 
that  the  Sun  has  changed  its  colour  and  lost  much  of  its  former 
size  and  brilliancy,  but  that  the  planets  are  not  in  the  same 
condition  they  were  in  before.  The  Third  Planet,  for  example, 
rotates  very  much  slower  than  it  used  to,  and  its  Moon  is  farther 
from  it.  The  result  is  that  the  Earth's  day  and  month  are  now 
the  same  length  (equal  to  about  57  of  our  days).  The  Moon 
still  shows  but  one  side  to  the  Earth,  and  the  Earth  now  shows 
but  one  side  to  the  Moon.  They  move  around  their  common 
centre  of  gravity  like  a  huge  dumb-bell  with  an  invisible 
handle.  The  Earth's  oceans  have  partly  frozen,  and  partly 
soaked  into  the  cold  interior.  And  its  atmosphere  has  congealed 
into  a  solid  snow-like  substance.  Like  its  Moon,  it  is  a  dead 
world,  waiting  for  the  inevitable  crash  that  is  to  bring  it  back 
to  some  other  form  of  life  and  usefulness. 

ORBITS  VARY 

It  may  be  as  well  to  mention  here  that  the  Earth's  orbit  is 
not  always  the  shape  it  is  now.  Sometimes  it  is  almost  circu- 
lar, while  at  other  periods  its  ellipticity  is  so  great  as  to  make 
the  Earth's  distance  from  the  Sun  vary  14,000,000  miles,  instead 
of  3,000,000  miles  as  at  present.  This  change  (combined  with 
the  Precession  of  the  Equinoxes,  and  various  geographical 
changes)  produces  considerable  variations  in  the  Earth's  climate 
at  long-distant  periods. 

If  we  regard  the  Solar  System  as  a  machine,  we  shall  see 
that  one  of  its  eccentrics  takes  109,000  years  to  go  once  around. 
Yet  there  are  people  on  Planet  Number  Three  who  think  that 
the  whole  Universe,  stars  and  all,  was  made  and  wound  up 
6,000  years  ago,  and  that  in  another  thousand  years  the  entire 
machine  will  have  run  down  and  worn  out ! 

ORBITS  ARE  TILTED 

Before  quitting  my  illustration  of  a  floating  Solar  System  it 
may  be  as  well  to  say  that,  besides  being  unreal  (a  mere  detail) 

10 


146     HOW  TO   KNOW  THE   STARRY   HEAVENS 

it  is  faulty,  even  for  an  illustration,  in  one  rather  important 
particular.  I  have  imagined  the  various  planets  to  be  floating 
on  the  surface  of  water.  Now  the  fact  is  that  the  plane,  or  level, 
or  surface,  on  which  one  planet  moves,  is  not  exactly  the  plane  or 
surface  on  which  the  others  move,  so  that,  if  the  Earth  be  re- 
garded as  floating  evenly  on  this  ecliptic  plane,  each  of  the 
other  planets  will  be  found  to  rise  a  little  above  its  surface  at 
one  part  of  its  orbit,  and  to  sink  a  little  below  it  at  another. 

This  is  the  reason  why  there  is  not  an  eclipse  of  the  Sun 
every  new-moon,  and  an  eclipse  of  the  Moon  every  full-moon. 
It  is  only  at  the  two  nodes,  where  the  plane  of  the  Moon's 
orbit  crosses  the  plane  of  the  Earth's  orbit,  that  eclipses  can 
occur.  For  elsewhere  the  Moon  passes  below  or  above  the 
straight  line  connecting  the  Earth  and  Sun.  The  same  is  true 
of  the  "  transits  "  and  "  occupations  "  of  the  planets  in  front 
of,  or  behind,  the  disc  of  the  Sun. 

SAME  POLE-STARS  EVERY  26,000  YEARS 

Now  suppose  that  the  first  time  we  visited  the  Solar  System 
we  took  a  complete  set  of  photographs  of  the  stars  as  seen  from 
Planet  Number  Three.  And  suppose  that  on  each  of  our  sub- 
sequent visits  we  take  a  fresh  set  of  photographs.  On  compar- 
ing these  we  shall  find  that  the  stars  themselves  are  about  the 
same,  but  that  what  should  be  called  (but  are  not)  their  celes- 
tial "  latitudes  and  longitudes  "  are  changing  all  the  time.  This 
is  of  course  due  to  the  fact  that  the  North  and  South  Poles  are 
all  the  time  slowly  circling  around  the  poles  of  the  Ecliptic.1 
But  if  we  make  many  visits  with  the  same  interval  between 
them,  we  shall  find  every  fourth  set  practically  the  same.  Thus 
the  first  and  fifth  sets  will  be  nearly  alike,  and  so  will  be  the 
second  and  sixth,  the  third  and  seventh,  and  the  fourth  and 
eighth.  The  reason  for  this  is  obvious,  for  when  the  poles 
have  "  wabbled  "  around  a  complete  revolution  they  come  back 
to  the  same  place  on  the  "  star-sphere,"  and  the  twelve  monthly 

1  See  North  Polar  Star  Chart. 


A   REELING   WORLD  147 

"  mansions  of  the  Sun "  again  coincide  with  the  same  constella- 
tions of  the  Zodiac. 

STARS  ARE  DRIFTING 

But  if  we  make  a  closer  examination  of  our  different  sets  of 
star-maps  we  shall  find  that  the  stars  themselves  are  slowly 
moving  about  in  space.  One  group  of  stars  is  drifting  in  this 
direction,  and  another  in  that.  Some  are  coming  nearer  to  us, 
and  others  are  receding.  Even  in  a  group  of  drifting  stars  the 
individuals  are  moving  slowly  around  among  themselves,  like 
the  motes  in  a  sunbeam.  This  slow  drift  will  eventually  make 
the  heavens  unrecognisable. 

THE  GREAT  PYRAMID 

It  is  interesting  to  know  that  when  the  Great  Pyramid  of 
Egypt  was  constructed,  the  North  Pole  of  the  Earth  was  nearly 
as  represented  in  our  third  return  to  the  planet.  The  building 
was  "oriented  "  (that  is,  adjusted  to  the  points  of  the  compass) 
by  making  a  descending  passage,  down  which  the  pole-star  of 
the  period  (Alpha  Draconis,  also  known  as  Thuban)  shone  at 
its  lowest  meridian  passage.  A  temporary  pool  of  water  was 
formed  some  distance  down  the  passage,  and  the  image  of  the 
star  reflected  up  a  second  (but  ascending)  passage.  The  result 
was  that  the  Pyramid  was  better  oriented  than  any  other  build- 
ing put  up  before  the  invention  of  the  telescope.  This  peculi- 
arity fixes  the  date  at  which  the  Pyramid  was  built  at  3,400 
B.  c.,  the  December  Sun  being  then  in  the  constellation  of 
Aquarius  (Number  11  in  Figure  77). 

The  long,  narrow,  but  lofty  gallery  which  formed  a  continua- 
tion of  the  ascending  passage  was  the  most  perfect  transit 
instrument  ever  made,  leaving  out  those  provided  with  magnify- 
ing instruments.  The  large  square  platform  at  the  top,  probably 
provided  with  corner-posts  and  observing-stations,  was  also  an 
excellent  arrangement  for  the  study  of  the  heavenly  bodies. 
By  the  long-continued  use  of  this  truncated  pyramid  the  science 


148     HOW  TO   KNOW   THE   STARRY   HEAVENS 

of  astronomy  might  have  been  largely  developed.  But  unfor- 
tunately it  was  covered  up  for  a  tomb  as  soon  as  its  childish 
astrological  purpose  had  been  served. 

THE  FIRST  POINT  OF  ARIES 

In  this  chapter  I  have  kept  track  of  the  planetary  motions 
by  referring  to  the  constellation  which  the  Sun  appears  to 
occupy  in  December.  In  practice  it  is  found  more  convenient 
to  note  what  stars  are  beyond  the  Sun  at  the  Spring  Equinox. 
Four  thousand  years  ago  the  March  Sun  was  in  the  constel- 
lation of  Taurus  (the  Bull).  The  Apis  worship  of  Egypt  was 
probably  founded  on  this  fact.  In  Europe  it  was  handed  down 
from  father  to  son  by  such  poetical  expressions  as  "  the  White 
Bull  opens  the  year  with  his  golden  horns."  Two  thousand 
years  later,  when  the  same  equinox  had  moved  back  to  the 
constellation  of  Aries  (the  Earn),  Jupiter  Ammon  was  repre- 
sented with  a  ram's  horns.  At  present  the  spring  equinox  is  in 
the  constellation  of  Pisces  (the  Fishes),  and  is  moving  in  the 
direction  of  Aquarius  (the  Water-Carrier). 

When  this  slow-moving  point  was  on  the  margin  between 
Aries  and  Pisces,  it  acquired  the  name  of  "  the  First  Point  of 
Aries,"  and  it  still  retains  this  now  misleading  name. 

It  may  perhaps  prevent  a  misunderstanding  if  we  regard  the 
Sun  (when  viewed  from  the  earth)  as  advancing  from  right  to 
left  into  a  fresh  "  house  "  every  month,  while  the  solar  houses 
themselves  are  hooked  on  to  the  equinoxes  and  solstices,  and 
therefore  drift  backward  at  an  extremely  slow  rate.  As  the 
constellations  of  stars  do  not  share  in  this  backward  drift,  the 
effect  is  as  though  the  celestial  bull,  ram,  fishes,  etc.,  were  drift- 
ing forward  from  one  house  to  another,  entering  a  fresh  "  house  " 
every  2,155  years.  Or  we  may  imagine  that  there  are  twelve 
picture  frames  fastened  on  to  the  Ecliptic,  and  that  the  Sun 
goes  from  one  frame  to  another  in  a  month,  going  from  right  to 
left.  It  will  thus  occupy  one  frame  every  January,  the  next 
one,  to  the  left,  every  February,  etc.  In  this  case  the  stars 


A   REELING  WORLD  149 

form  the  framed  pictures,  and  they  very  slowly  drift  in  the 
same  direction,  staying  in  each  frame  a  little  more  than  2,000 
years. 

DECLINATION  AND  RIGHT  ASCENSION 

In  mapping  out  the  heavens  it  has  been  found  convenient  to 
divide  them  into  360  "  longitudinal "  degrees  (or  into  24  hours), 
by  imaginary  lines  passing  through  the  Celestial  Equator  to  the 
poles.  The  zero  of  these  divisions  crosses  the  Equator  and 
Ecliptic  where  they  cross  each  other  at  the  so-called  First 
Point  of  Aries.  The  same  number  of  "  latitudinal "  degrees  are 
used  to  denote  the  distance  of  any  celestial  object  north  or 
south  of  the  Celestial  Equator.  These  celestial  circles  are 
identical  with  the  terrestrial  longitudes  and  latitudes  which  are 
used  to  denote  the  position  of  any  place  on  our  Earth.  For 
convenience,  the  earthly  longitudes  have  their  zero  at  Green- 
wich, England. 

For  the  sake  of  simplicity,  let  us  suppose,  for  a  time,  that  we 
are  living  at  Greenwich.  We  shall  find  that,  owing  to  the 
daily  rotation  of  the  Earth,  the  First  Point  of  Aries  appears  to 
cross  the  local  meridian  every  twenty-three  hours  and  fifty-six 
minutes.  At  the  moment  of  crossing,  the  two  longitudinal 
zeros  coincide,  and  all  the  stars  which  are  on  the  local  (Green- 
wich) meridian  at  that  instant  might  be  said  to  have  a  longi- 
tude of  0°,  or  of  0  hours.  To  prevent  confusion,  however,  they 
are  said  to  have  a  Right  Ascension  of  0°,  or  of  0  hours. 

The  number  of  hours  and  minutes  which  elapse  before  a 
certain  star  passes  the  same  (Greenwich)  meridian  is  termed 
the  right  ascension  of  that  particular  star.  For  example,  a  star 
which  is  on  the  meridian  of  Greenwich  six  hours  later  than  the 
Equinoctial  Colure  which  contains  the  First  Point  of  Aries  is 
said  to  have  a  right  ascension  of  6  hours,  or  of  90°. 

The  number  of  degrees  separating  a  star,  etc.,  from  the  celestial 
equator  might  be  known  as  its  latitude,  but  for  convenience  it  is 
really  known  as  its  North  (or  South)  Declination.  For  exam- 
ple, if  it  is  20°  south  of  the  Celestial  Equator,  it  is  said  to  have 
a  south  declination  of  20°. 


150     HOW   TO   KNOW   THE   STARRY   HEAVENS 

By  measuring  the  position  of  a  star  or  other  object  in  these 
two  ways,  its  position  in  the  heavens  can  be  registered  with  an 
accuracy  which  is  limited  only  by  the  imperfections  of  the 
observers  and  their  instruments.1 

CELESTIAL  LATITUDES  AND  LONGITUDES 

It  will  be  noticed  that  both  of  these  celestial  measurements 
start  from  zeros  which  move  around  with  the  Precession  of  the 
Equinoxes.  The  result  is  that  both  the  declination  and  right 
ascension  of  every  star  change  very  slowly  as  the  Earth's  axis 
reels  round  the  poles  of  the  ecliptic.  To  avoid  this  objection, 
it  would  be  necessary  to  use  latitudes  and  longitudes  based  on 
the  ecliptic  and  its  poles.  For  most  purposes,  however,  the 
equatorial  and  polar  measurements  are  most  convenient. 

IMPROVED  EQUATORIAL  INSTRUMENT 

At  the  close  of  the  First  Chapter  I  described  a  very  primi- 
tive form  of  equatorial  instrument  which  could  be  used  to 
"  place  "  the  various  heavenly  bodies,  and  to  follow  their  motions, 
both  real  and  apparent  (see  Figure  6). 

This  instrument  can  be  greatly  improved,  so  far  as  right 
ascension  is  concerned,  by  fixing  a  cog-wheel  with  24  teeth  at 
one  end  of  the  polar  axis,  and  placing  a  spring  so  that  it  presses 
against  one  of  the  teeth.  On  turning  the  telescope  or  pointer 
around  toward  the  east,  the  spring  will  click  off  the  hours  of 
right  ascension.  When  the  pointer  is  directed  to  Alpheratz, 
the  upper  left-hand  star  in  the  Square  of  Pegasus,  or  at  the 
most  westerly  star  in  the  W  of  Cassiopeia,  it  is  at  the  zero  of 
right  ascension,  which  passes  through  the  First  Point  of  Aries. 
One  click  will  bring  it  to  the  right  ascension  of  Mirach,  in 

1  When  the  observer  does  not  live  at  Greenwich,  he  has  to  allow,  in  all  his 
calculations,  for  the  difference  in  time  and  latitude.  For,  on  the  one  hand,  it  is 
obvious  that  if  the  First  Point  of  Aries  is  on  the  meridian  of  Greenwich,  it  can- 
not at  the  same  time  be  on  the  meridian  of  New  York  or  San  Francisco.  And, 
on  the  other  hand,  it  is  also  obvious  that  a  star  that  is  on  the  zenith  at  Greenwich 
is  a  long  way  from  the  zenith  of  Cape  Town, 


A   REELING  WORLD  151 

Andromeda.  Another  click  will  bring  it  in  a  line  with  Alpha 
Arietes,  also  known  as  Hamal.  Between  the  fifth  and  sixth 
clicks  it  will  pass  Kigel  and  Betelgeuse.  At  the  tenth  click 
Kegulus  will  be  in  line.  Soon  after  the  fourteenth,  Arcturus 
will  line  up.  Between  the  eighteenth  and  nineteenth,  Vega 
will  be  passed;  and  at  the  twenty-third  click  the  two  right- 
hand  stars  of  the  Square  of  Pegasus  will  fall  in  line.  Each 
click  will  of  course  represent  one  hour  (or  15°)  of  right  ascen- 
sion (see  Star  Charts  and  Keys). 

The  same  instrument  can  also  be  greatly  improved,  so  far  as 
north  and  south  declination  is  concerned,  by  fixing  a  similar 
cog-wheel  and  spring  where  the  telescope  or  pointer  turns  on 
the  polar  axis.  On  turning  the  telescope  or  pointer  to  the 
north  or  south,  the  spring  will  click  off  every  15  degrees.  When 
it  points  to  Mintaka,  the  northwestern  star  in  the  Belt  of  Orion, 
it  will  be  on  the  equatorial  zero.  One  click  to  the  north  will 
bring  it  to  the  declination  of  Aldebaran,  in  Taurus,  and  of  the 
southern  stars  in  the  Square  of  Pegasus.  One  click  to  the 
south  of  the  equator  will  almost  bring  Sirius  in  line.  Three  to 
the  north  will  give  the  equatorial  distance  of  Capella  and  Arided. 
Two  clicks  to  the  south  will  bring  it  to  the  declination  of 
Fomalhaut  (see  Star  Charts  and  Keys). 

By  having  three  times  as  many  teeth  in  the  two  cog-wheels, 
every  click  will  represent  5  degrees,  and  the  positions  of  the 
stars,  etc.,  can  be  more  accurately  ascertained.  Many  modern 
telescopes  have  wheels  or  circles  so  finely  divided  that  the 
divisions  have  to  be  read  off  with  the  help  of  a  microscope. 
And  their  polar  axis  is  turned  by  clockwork,  so  as  to  keep  up 
with  the  (apparent)  diurnal  motion  of  the  heavens  (see  Figures 
43,  46,  and  79). 

The  right  ascension  and  north  and  south  declination  of 
"  many  ten  thousands  "  of  stars  are  accurately  recorded  in  star 
catalogues.  By  turning  the  two  axes  of  an  equatorial  telescope 
till  the  index  fingers  point  to  the  position  of  a  star  as  given  in 
these  catalogues,  it  is  at  once  placed  in  the  centre  of  the  tele- 
scope's field  of  view.  This  can  be  done  even  in  the  daytime, 


152     HOW  TO   KNOW  THE   STARRY   HEAVENS 

and  some  of  the  stars  and  planets  can  be  found  and  observed 
telescopically  while  the  Sun  is  above  the  horizon. 

MERIDIAN  INSTRUMENTS 

If  the  instrument  described  above  is  pointed  to  the  zenith 
(or  point  overhead),  and  the  polar  axis  is  then  permanently 
clamped,  the  range  of  the  telescope  is  reduced  to  a  straight 
north-and-south  line,  forming  one  half  of  a  great  circle  of  the 
heavens.  It  can  be  pointed  to  the  northern  horizon,  to  the 
polar  axis  of  the  heavens,  to  the  point  overhead,  to  the  southern 
horizon,  or  to  any  intermediate  point.  But  it  cannot  be  directed 
to  any  part  of  the  heavens  either  east  or  west  of  the  local  merid- 
ian. In  order  to  observe  any  heavenly  body  with  this  crippled 
equatorial  it  is  necessary  to  wait  until  the  apparent  diurnal 
motion  of  the  heavens  brings  the  object  to  the  meridian.  Sup- 
posing that  we  are  still  at  Greenwich,  let  us  wait  till  the  zero 
of  right  ascension  passes  the  field  of  view,  and  then  turn  the 
hands  of  a  24-hour  clock  till  they  point  to  24  o'clock.  The 
clock  will  then  register  what  is  known  as  sidereal  time.1 

As  the  northeast  star  in  the  Square  of  Pegasus  is  at  present 
close  to  that  zero,  we  can  start  the  sidereal  clock  when  that 
star  crosses  the  field  of  view.  It  will  be  found  that  the  star 
known  as  Mirach  passes  at  one  o'clock  (sidereal),  Alpha  Arietes 
at  two,  Regulus  at  ten,  and  Arcturus  at  fourteen  o'clock.  These 
stars  are  therefore  said  to  have  so  many  hours  of  right  ascen- 
sion, and  by  measuring  the  number  of  degrees  they  are  north  or 
south  of  the  Equator  (90°  from  each  of  the  Poles)  we  can  ascer- 
tain their  north  or  south  declination. 

These  two  classes  of  observation  are  so  important  that  greater 
precision  is  obtained  by  using  a  special  instrument,  with  no 
polar  axis,  but  with  large  and  carefully  constructed  declination 
circles.  This  instrument  is  called  a  Meridian  Circle  (see  Fig- 
ure 80).  It  is  mounted  on  a  horizontal  axis  lying  east  and 
west.  This  axis  rests  on  the  top  of  two  massive  piers,  so  that 

1  It  should  be  regulated  so  as  to  gain  four  minutes  in  24  hours  of  solar  time. 


A   REELING   WORLD 


153 


the  arrangement  is  similar  to  that  of  a  cannon  mounted  on  its 
carriage. 

When  this  meridian  circle  is  being  used  as  a  transit  instru- 
ment, the  exact   sidereal  time  when  a  celestial  body  crosses 
the  centre  of   the    field   of 
view   is  observed,  and  the 
star  or  planet  is  said  to  have 
a  right   ascension   of  that 
number  of  hours,  minutes, 
and  seconds. 

When  this  meridian  cir- 
cle is  used  as  a  mural  circle, 
to  find  the  distances  of 
stars,  etc.,  from  the  Celestial 
Poles  or  Equator,  the  clock 
is  disregarded,  but  their 
distances  from  the  local 
zenith  are  carefully  read 
off  on  the  large  declination 
circles  attached  to  the  axis 
of  the  telescope.  As  the 
distances  of  the  local  zenith 
from  the  Celestial  Pole  and 
Equator  have  been  pre- 
viously ascertained,  the  ze- 
nith distance  of  any  object 
can  easily  be  turned  into 
either  "  polar  distance  "  or 
declination  (north  or  south), 
as  may  be  preferred. 


ALT-AZIMUTH  TELESCOPE 


FIG.  81.  —  ALT-AZIMUTH  MOUNTING  FOK 
SMALL  TELESCOPE 

One  of  the  screw  motions  changes  the  altitude, 
and  the  other,  the  azimuth  or  point  of  the  compass. 


There  is  another  form  of 
telescope-mounting    known 

as  the  alt-azimuth.     As  its  name  implies,  this  gives  two  mo- 
tions, one  around  a  perpendicular  axis,  changing  the  "  azimuth  " 


154    HOW  TO   KNOW  THE   STARRY  HEAVENS 

or  point  of  the  compass,  and  the  other  around  a  horizontal  axis, 
changing  the  "  altitude  "  (see  Figure  81).  This  form  of  mount- 
ing, though  very  handy  for  terrestrial  purposes,  is  inconvenient 
for  celestial  objects,  which  almost  always  move  at  an  angle 
with  the  horizon.  For  any  one  living  at  the  North  or  South 
Pole,  however,  it  is  the  best  form,  as  the  perpendicular  axis 
becomes  a  polar  axis,  and  the  instrument  is  transformed  into 
an  equatorial. 

USES  OF  INSTRUMENTS 

It  will  be  seen  from  the  above  that  both  the  alt-azimuth  and 
the  equatorial  are  capable  of  being  directed  to  any  part  of  the 
sky,  and  can  be  made  to  keep  a  celestial  object  in  view  for  any 
length  of  time,  provided  that  it  is  above  the  horizon.  The  alt- 
azimuth is,  however,  difficult  to  use  for  most"  astronomical  pur- 
poses, as  the  diurnal  motions  can  be  followed  only  by  turning 
both  axes  at  the  same  time.  The  equatorial  form  is  therefore 
commonly  used  for  general  observational  purposes.  The  various 
attachments  used  for  celestial  photography  and  spectroscopy 
are  all  applied  to  telescopes  so  mounted,  with  a  clockwork 
movement  to  the  polar  axis,  to  neutralise  the  apparent  motions 
due  to  the  Earth's  rotation. 

The  meridian  circle,  however,  moves  solely  on  the  local  me- 
ridian, and  no  celestial  object  can  be  seen  through  its  telescope 
except  for  a  few  seconds  every  24  hours,  as  it  crosses  the  field 
of  view  from  east  to  west.  Of  course  such  a  telescope  is  of  no 
use  for  "  star-gazing."  Its  chief  purpose  is  for  measuring  the 
polar  distances  and]  hour  angles  of  the  heavenly  bodies  on  the 
celestial  "  sphere." 


CHAPTER  XIII 

KEPLER'S  THREE  LAWS 

"  In  the  phenomena  presented  to  him,  man  must  [have  early  noticed]  two 
kinds  of  relation.  Some  things  show  themselves  with  other  things,  and  some 
things  follow  other  things.  These  two  kinds  of  relation  we  call  relations  of  co- 
existence, and  relations  of  succession  or  sequence.  Since  what  continues  is  not  so 
apt  to  attract  our  attention  as  what  changes,  it  is  probable  that  the  first  of  these 
two  relations  to  be  noticed  is  that  of  succession.  .  .  .  Now  of  the  sequences  which 
we  notice  in  external  nature,  some  are  variable,  .  .  .  while  some  are  invari- 
able. ...  As  to  these  invariable  sequences,  which  we  properly  call  con-sequences,  we 
give  a  name  to  the  causal  connection,  between  what  we  apprehend  as  effect  and 
what  we  assume  as  cause,  by  calling  it  a  Law  of  Nature.  ...  In  original  meaning 
the  word  law  refers  to  human  will,  and  is  the  name  given  to  a  command  or  rule 
of  conduct  imposed  by  a  superior  upon  an  inferior,  as  by  a  sovereign  or  State 
upon  those  subject  to  it.  At  first  the  word  law  doubtless  referred  only  to  human 
law.  But  when,  later  in  intellectual  development,  men  came  to  note  invariable 
co-existences  and  sequences  in  the  relations  of  external  things,  they  were,  of  the 
mental  necessity  already  spoken  of,  compelled  to  assume  as  cause  a  will  superior* 
to  human  will,  and,  adopting  the  word  they  were  wont  to  use  for  the  highest 
expression  of  human  will,  called  them  laws  of  Nature.  Whatever  we  observe  as 
an  invariable  relation  of  things,  of  which  in  the  last  analysis  we  can  only  affirm 
that  '  it  is  always  so/  we  call  a  law  of  Nature."  —  Henry  George. 

UNCHANGING  LAWS 

PEEHAPS  the  most  striking  thing  in  the  study  of  astronomy 
is  the  fact  that  everywhere  there  are  evidences  that  the 
whole  Universe  is  absolutely  ruled  by  unchanging  laws.  There 
is  no  such  thing  as  chance.  Hesitation  and  uncertainty  are 
unknown.  Knowing  the  forces  which  are  at  work,  the  as- 
tronomer can  calculate  with  certainty  the  positions  and  move- 
ments of  the  other  planets  in  our  System  for  thousands  of 
years  to  come.  And  even  beyond  our  System,  at  distances 
which  are  too  vast  for  the  imagination  itself  to  fathom,  he  can 


156    HOW  TO  KNOW  THE   STARRY   HEAVENS 

follow  up  the  movements  of  some  of  the  stellar  groups  and 
predict  their  future  positions  and  relations. 

In  order  to  do  these  things  he  has  to  cast  aside  all  ideas  of 
chance  or  miracle,  and  rely  entirely  on  the  immutability  of  the 
laws  of  Nature. 

These  natural  laws  must  not  be  confounded  with  mere 
human  decrees,  which  are  all  more  or  less  arbitrary,  change- 
able, and  local,  as  well  as  ineffective.  Human  laws  may  to-day 
decree  that  in  a  certain  country  one  kind  of  stealing  shall  be 
legal  and  praiseworthy,  while  another  kind  of  stealing  shall 
be  criminal  and  punishable ;  and  to-morrow  the  decree  may  be 
reversed,  if  the  rulers  think  fit.  For  example,  in  Old  Testa- 
ment times  it  was  against  the  law  to  charge  interest  (usury). 
At  present  it  is  a  legal  and  even  respectable  custom.  Some 
day  it  may  again  become  a  crime. 

But  natural  laws  exist  without  any  decree  being  promulgated. 
They  are  not  the  creation  or  invention  of  mind,  but  the  inevi- 
table result  of  mathematical  necessity.  They  rule  alike  the 
entire  Universe.  All  matter  is  absolutely  subject  to  them, 
whether  it  be  dead  or  living.  They  are  the  same  to-day  as  they 
were  a  billion  or  a  trillion  years  ago,  and  they  will  be  the  same 
a  billion  or  a  trillion  years  to  come.  For  example,  2  +  2  =  4, 
always  and  everywhere.  Omnipotence  itself  could  not  invent 
or  change  this  natural  law.  It  would  be  just  as  true  if  nothing 
existed. 

George  N.  Lowe  says  of  Natural  Law  — 

"  The  Law  no  contract  knows,  no  lease 
Of  being,  waning  past  its  prime  — 
She  moves  but  in  Eternities  ; 
Supreme,  she  hath  no  need  of  Time." 

Dr.  J.  W.  Draper,  in  his  "  Conflict  Between  Religion  and 
Science,"  says : 

"  Astronomical  predictions  of  all  kinds  depend  on  the  admission  of 
this  fact  —  that  there  never  has  been,  and  never  will  be,  any  interven- 
tion in  the  operation  of  natural  laws.  The  scientific  philosopher 


KEPLER'S  THREE  LAWS  157 

affirms  that  the  condition  of  the  [Universe]  at  any  given  moment  is 
the  direct  result  of  its  condition  in  the  preceding  moment,  and  the 
direct  cause  of  its  condition  in  the  subsequent  moment.  Law  and 
chance  are  only  different  names  for  mechanical  necessity." 

KEPLER'S  LAWS 

It  was  Kepler l  who  first  discovered  the  music  which  regu- 
lates the  whirling  of  the  worlds  through  space.  His  explana- 
tions of  the  planetary  motions  are  known  as  Kepler's  Three 
Laws.  They  are  as  follows: 

I.  The  orbit  of  every  planet  is  an  ellipse,  with  the  Sun  in  one  of 
its  foci. 

II.  An  imaginary  line  drawn  from  the  Sun  to  a  planet  will  sweep 
over  equal  areas  in  equal  times. 

III.  The  squares  of  the  numbers  representing  the  periodic  times 
of  the  planets  vary  as  the  cubes  of  the  numbers  representing  their 
mean  or  average  distances. 

These  laws  are  easily  stated,  and,  to  one  who  understands 
the  meaning  of  the  terms  employed,  are  not  difficult  to  com- 
prehend. To  us,  indeed,  they  seem  as  obvious,  natural,  and 
inevitable  as  the  statement  that  two  and  two  make  four. 

Yet  it  was  not  always  so.  For  thousands  of  years  intelligent 
astronomers,  of  many  nationalities,  were  seeking  diligently  for 
these  three  laws,  but  never  found  them.  After  every  other 
theory,  probable  and  improbable,  had  failed  to  explain  the 
apparent  motions  of  the  planets,  Kepler  found  that  the  move- 
ments of  Mars  could  all  be  accounted  for  on  the  theory  that 
each  planet  moves  in  an  elliptical  orbit  around  the  Sun  with 
a  velocity  varying  with  its  distance  from  that  body.  Let  us 
examine  his  conclusions. 

FIRST   LAW 
"  The  orbit  of  every  planet  is  an  ellipse,  with  the  Sun  in  one  of  the  foci." 

In  order  to  comprehend  this  we  must  clearly  understand 
what  an  ellipse  really  is.  • 

1  Born  in  1571 ;  died  in  1630. 


158    HOW  TO   KNOW   THE   STARRY   HEAVENS 

Get  a  piece  of  white  pasteboard.  Stick  a  pin  upright  on  one 
side  of  it,  near  the  centre.  Tie  the  two  ends  of  a  piece  of  thread 
together,  so  as  to  make  a  loop  three  inches  long.  Pass  one  end 
of  this  loop  over  the  pin,  and  insert  the  point  of  a  pencil  through 
the  other  end  of  it.  Let  the  pencil-point  touch  the  card,  and 
move  it  around  the  centre-pin  so  as  to  make  a  six-inch  circle. 
This  circle  is  really  an  ellipse  with  no  eccentricity. 

Now  stick  two  more  pins  in  the  pasteboard,  one  on  each  side 
of  the  first,  so  that  each  is  the  eighth  of  an  inch  from  the 
centre  one.  Pass  the  loop  over  them  all,  insert  the  point  of  a 
pencil  as  before,  and  again  move  it  around  on  the  card,  keep- 
ing the  string  stretched  all  the  time.  The  resulting  figure 
looks  like  a  circle,  but  it  will  be  found,  on  measuring  it,  that 
it  is  a  trifle  narrower  one  way  than  another.  And  instead  of 
having  one  centre,  or  focus,  it  has  two  eccentric  foci,  each  one 
being  an  eighth  of  an  inch  away  from  the  centre  of  the  figure. 
The  apparent  circle  is  really  an  ellipse  of  small  eccentricity. 

By  increasing  the  interval  between  the  two  pins,  or  foci,  we 
can  produce  a  great  variety  of  ellipses.  When  the  two  foci 
are  two  inches  apart,  the  resulting  figure,  instead  of  being  a 
six-inch  circle,  is  an  ellipse  measuring  4  inches  one  way,  and 
3^  inches  the  other  way.  As  each  focus  is  some  distance  from 
the  centre,  the  figure  is  termed  an  ellipse  of  great  eccentricity 
(see  Figure  82). 

We  are  now  in  a  position  to  understand  Kepler's  First  Law, 
that  each  planet  moves  in  an  ellipse,  with  the  Sun  in  one  of 
the  foci.1 

SECOND  LAW 

"  An  imaginary  line  drawn  from  the  Sun  to  a  planet  will  sweep  over  equal 
areas  in  equal  times." 

The  simplest  form  in  which  we  can  study  this  law  is  where 
the  elliptical  orbit  has  no  eccentricity ;  that  is  to  say,  when 

1  In  most  astronomical  calculations,  one  focus  alone  is  of  importance,  the 
other  one  being  placed  on  the  shelf,  along  with  the  so-called  "  fourth  dimension 
of  space." 


FIG.  82.  —  DRAWING  AN  ELLIPSE 


KEPLER'S  THREE  LAWS  159 

the  ellipse  is  a  true  circle,  with  the  two  foci  together  at  the 
centre. 

An  ordinary  carriage-wheel  with  twelve  spokes  will  do  to 
illustrate  this  form  of  ellipse.  The  spokes  of  such  a  wheel 
are  all  of  the  same  length  and  are  the  same  distance  apart. 


FIG.  83. — AN  ELLIPTICAL  ORBIT,  DIVIDED  INTO  TWELVE 

MONTHLY  PARTS 

This  ellipse  is  more  eccentric  than  the  majority  of  planetary  orbits, 
but  is  less  so  than  the  orbits  of  comets. 

The  spaces  between  them  are  therefore  all  the  same  size ;  that 
is,  they  all  contain  the  same  number  of  square  inches. 

If  an  insect  should  crawl  straight  along  the  tire  of  this 
wheel  at  a  uniform  speed,  it  is  obvious  that  it  would  always 
take  the  same  interval  of  time  to  go  from  one  spoke  to  another, 
and  an  imaginary  line  between  the  insect  and  the  centre  of  the 
wheel  would  always  sweep  over  the  same  number  of  square 
inches  per  second. 

It  is  the  same  in  the  case  of  a  planet.  "  An  imaginary  line 
drawn  from  the  Sun  to  a  planet  will  sweep  over  equal  areas  in 


160     HOW  TO   KNOW   THE   STARRY   HEAVENS 

equal  times."  So  when  this  line  has  swept  over  one  twelfth  of 
the  area  of  the  orbit,  one  twelfth-part  of  the  planet's  revolu- 
tionary period  has  passed  away. 

When  the  wheel  is  not  a  circular  ellipse,  but  an  eccentric 
one,  the  details  are  rather  different,  though  the  law  is  the  same. 
In  this  case  the  spokes  radiate  from  one  of  the  foci  instead  of 
from  the  centre  (see  Figure  83).  And  if  the  area  (or  number 
of  square  inches)  between  each  of  the  spokes  is  to  remain  the 
same,  it  follows  that  the  long  spokes  must  be  set  closer  to- 
gether than  the  short  ones. 

From  this  it  is  evident  that  if  the  insect  wishes  to  go  from 
one  spoke  to  another  in  the  same  interval  of  time,  it  must 
crawl  slowly  when  on  that  part  of  the  tire  where  the  spokes 
are  long  and  close  together,  and  increase  its  speed  as  it  goes 
to  where  the  spokes  are  short  and  far  apart. 

It  is  the  same  in  the  case  of  a  planet.  The  speed  increases 
and  diminishes  according  to  the  planet's  distance  from  the 
Sun,  so  that,  in  this  case  too,  "  an  imaginary  line  drawn  from  the 
Sun  to  a  planet  will  sweep  over  equal  areas  in  equal  times." 
And,  as  in  the  case  of  a  circular  orbit,  when  this  line  (called  a 
radius  vector)  has  swept  over  one  twelfth  of  the  area  of  the 
orbit,  one  twelfth-^&rt  of  the  planet's  revolutionary  period  has 
passed  away. 

Figure  83  is  an  exaggerated  view  of  the  Earth's  orbit,  with 
12  "spokes"  (or  radius  vectors)  radiating  from  the  Sun,  which 
occupies  one  of  the  foci.  The  Earth  goes  from  one  "  spoke  " 
to  another  in  an  average  month  (about  30J  days),  and  there- 
fore travels  faster  when  nearest  to  the  Sun  (in  perihelion) 
than  it  does  six  months  later,  when  it  is  at  its  greatest  distance 
from  the  Sun  (in  aphelion).1 

1  If  any  orbit  is  correctly  marked  out  in  pasteboard,  and  the  twelve  wedge- 
shaped  pieces  are  then  cut  out,  they  will  all  be  found  to  weigh  alike,  because, 
though  different  in  shape,  they  all  have  the  same  size  or  area. 


KEPLER'S  THREE   LAWS  161 


THIRD  LAW 

"  The  squares  of  the  numbers  representing  the  periodic  times  of  the  planets 
vary  as  the  cubes  of  the  numbers  representing  their  mean  or  average  distances." 

The  law  which  we  have  just  discussed  deals  with  the  vary- 
ing velocity  of  one  planet  alone.  The  one  we  have  now  come 
to  compares  together  the  periodic  times  of  different  planets, 
and  shows  that  they  have  a  definite  relation  to  the  distances  of 
the  planets  from  the  Sun.  It  also  enables  us  to  compare  the 
mean  or  average  velocity  of  one  planet  with  the  mean  or 
average  velocity  of  any  other  planet.  The  periodic  time  of  a 
planet  is  of  course  the  time  it  takes  to  go  once  around  the  Sun. 
In  other  words,  it  is  the  length  of  the  planet's  year. 

The  outer  planets  have  a  longer  journey  to  make  than  the 
inner  ones.  And  they  move  with  less  rapidity.  Consequently 
their  years  or  periodic  times  are  considerably  longer. 

Observation  has  shown  that  Jupiter,  which  is  5.2  times  as 
far  from  the  Sun  as  our  Earth,  takes  11.86  times  as  long  to 
complete  a  revolution.  In  other  words,  Jupiter's  journey  is  a 
little  over  5  times  as  long  as  ours,  but  his  year  is  nearly  12 
times  as  long. 

Let  us  see  how  Kepler's  Third  Law  fits  in  with  this.  If 
we  square  the  periodic  times,  1  and  11.86,  we  get  (omitting 
fractions)  the  numbers  1  and  140.  And  if  we  cube  the  distances, 
1  and  5.2,  we  get  1  and  140.  These  results  are  identical. 

In  the  above  example  the  smaller  period  and  distance  are 
both  represented  by  unity  (1)  to  save  figuring.  Here  is  another 
example,  given  in  miles  and  days.  The  distances  of  Mercury 
and  Venus  from  the  Sun  are  46,000,000  and  67,000,000  miles 
respectively.  When  these  numbers  are  cubed,  the  latter  ex- 
ceeds the  former  6J  times.  Their  periods  of  revolution  are 
88  and  225  days,  respectively.  When  these  numbers  are 
squared,  the  latter  exceeds  the  former  6J  times.  These  results 
(like  those  in  the  Earth-Jupiter  example)  are  identical,  and  if 
we  perform  the  operation  with  any  two  planets  we  get  a  similar 

11 


162     HOW  TO  KNOW  THE   STARRY   HEAVENS 

result.  So  we  find  that  "  the  squares  of  the  periodic  times  are 
in  the  same  proportion  as  the  cubes  of  the  distances." 

The  velocity  of  a  planet  is  easily  computed  when  we  know 
the  size  of  its  orbit  and  the  length  of  its  year.  Jupiter  has 
5.2  times  as  far  to  go  as  the  Earth,  so  that  if  he  moved  with 
the  same  velocity  he  would  complete  a  revolution  in  5.2  of  our 
years.  As  his  year  is  2.28  times  as  long  as  that,  the  average 
velocity  of  the  Earth  in  its  orbit  is  evidently  2.28  times  that  of 
Jupiter.  Accordingly  we  find  that  while  Jupiter  travels  486 
miles  per  minute,  our  Earth  travels  1,108  miles  in  the  same 
interval  of  time  (486  x  2.28  =  1108).1 

Kepler  not  only  discovered  that  the  planets  move  in  ellipti- 
cal orbits  (according  to  the  laws  which  he  formulated),  but  also 
convinced  himself  that  their  notions  were  due  to  mutual 
attraction  between  them  and  the  Sun.  Unfortunately  he  was 
not  able  to  demonstrate  this.  It  was,  with  him,  a  probable  but 
unproved  theory. 

1  It  was  afterward  discovered  by  Newton  that  Kepler's  Third  Law  is  the 
result  of  the  solar  attraction  alone,  so  that  it  is  only  strictly  correct  in  the  case  of 
planets  consisting  of  single  infinitesimal  particles.  Where  the  planet  is  large,  its 
attraction  must  be  allowed  for  as  well.  The  correction  does  not  make  much 
difference  in  the  result,  so  far  as  the  above  examples  are  concerned,  but  it  enables 
us  to  extend  the  law  to  the  satellites  of  planets  and  to  stellar  systems.  The  de- 
tails of  this  correction  will  be  found  in  most  modern  text-books. 


CHAPTER  XIV 

GALILEO'S   LAWS   OF  MOTION 

"  When  we  find  in  Nature  certain  invariable  sequences,  whose  cause  of  being 
transcends  the  power  of  the  will  testified  to  by  our  own  consciousness ;  such,  for 
instance,  as  that  stones  and  apples  always  fall  toward  the  Earth ;  that  the  square 
of  a  hypothenuse  is  always  equal  to  the  sum  of  the  squares  of  its  base  and  per- 
pendicular ;  .  .  .  and,  so  on  through  the  list  of  invariable  sequences  that  these 
will  suggest,  we  say  —  for  it  is  really  all  that  we  can  say  —  that  these  sequences 
are  invariable  because  they  belong  to  the  order  or  system  of  Nature ;  or,  in 
short,  that  they  are  Laws  of  Nature.  .  .  . 

Why  is  it  that  some  things  always  co-exist  with  other  things  ?  The  Moham- 
medan will  answer :  '  It  is  the  will  of  God.'  The  man  of  our  western  civilisation 
will  answer :  '  It  is  a  law  of  Nature.'  The  phrase  is  different,  but  the  answer 
one."  —  Henry  George. 

ARISTOTLE'S  THEORY  OF  MOTION 

FEOM  very  ancient  times  men  have  tried  to  find  the  laws 
which  regulate  motion,  both  on  Earth  and  in  the  heavens. 
Aristotle,  the  founder  of  science,  gave  the  following  as  his  view 
of  the  subject : 

"All  simple  motion  must  be  rectilinear  or  circular,  either  to  a 
centre  or  from*  a  centre,  each  of  which  is  rectilinear,  or  about  a  centre. 
It  is  natural  for  two  of  the  elements  —  earth  and  water  —  to  tend  to 
a  centre  ;  two  — air  and  fire  —  which  are  light,  to  tend/rom  a  centre. 
As  the  motion  of  all  terrestrial  elements  is  therefore  rectilinear,  it 
seems  reasonable  that  the  celestial  bodies,  which  are  of  a  different 
nature,  should  have  the  only  other  simple  motion  possible,  namely, 
circular  motion." 

This  reasoning  sounds  strange  to-day,  though  it  held  its  own 
until  the  time  of  Kepler.  That  philosopher  finally  threw  aside 
the  celestial  part  of  it,  and  substituted  his  laws  of  planetary 
motion,  dealt  with  in  the  preceding  chapter. 


164     HOW  TO   KNOW  THE   STARRY   HEAVENS 

GALILEO'S  LAWS 

The  terrestrial  part  of  Aristotle's  law  also  had  to  give  way 
about  the  same  time.  Galileo l  discovered  that  on  our  Earth  all 
bodies  move,  or  abstain  from  moving,  according  to  the  follow- 
ing laws,  which  were  put  in  their  present  shape  by  Sir  Isaac 
Newton. 

I.  Every  body  continues  in  its  state  of  rest,  or  of  uniform 
motion  in  a  straight  line,  unless  it  is  compelled  to  change  that 
state  by  forces  impressed  thereon. 

II.  The    alteration  of   motion   is  ever   proportional   to  the 
motive  force  impressed,  and  is  made  in  the  direction  of  the 
straight  line  in  which  that  force  is  impressed. 

III.  To  every  action  there  is  always  opposed  an  equal  re- 
action, or  the  mutual  actions  of  two  bodies  are  always  equal 
and  directed  to  contrary  parts. 

FIRST  AND  SECOND  LAWS  OF  MOTION 

One  result  of  the  first  law  of  motion  is  that  a  body  which  is 
at  rest  will  stay  where  it  is  until  it  is  acted  upon  by  some 
force.  This  is  obvious  enough,  but  there  is  another  result 
which  does  not  seem  so  clear.  It  is  that  if  a  body  be  once  set 
in  motion  with  a  certain  velocity  it  will  (if  not  acted  on  by  any 
other  force)  continue  to  move  for  ever  in  a  straight  line  and  at 
the  same  velocity.  This  tendency  to  resist  change  (either  of 
rest  or  motion)  is  known  as  inertia. 

We  have  no  means  of  testing  this  first  law  by  itself,  for  when 
we  examine  an  object  which  appears  to  be  at  rest  we  find  that 
the  attraction  of  the  Earth  is  holding  it  in  its  place ;  and  when 
we  set  anything  in  motion  we  are  unable  to  prevent  other  forces 
from  acting  on  it.  If  we  fire  a  bullet  out  of  a  gun,  it  is  acted 
on  not  only  by  the  initial  impulse  of  the  exploding  powder 
behind  it,  but  also  by  the  continuous  attraction  of  the  Earth 
beneath  it.  It  has  also  to  contend  with  the  continuous  resist- 
ance of  the  air  in  front  of  it.  If  any  wind  is  blowing,  it  may 

l  Born  in  1564;  died  in  1642. 


GALILEO'S  LAWS   OF  MOTION  165 

be  affected  by  a  lateral  force  as  well.  All  of  these  modify  and 
finally  overcome  what  (according  to  this  law)  would  otherwise 
have  been  a  uniform  motion  in  a  straight  line. 

But,  according  to  the  second  law  of  motion,  any  secondary 
force  impressed  on  the  flying  object  produces  a  change  of  mo- 
tion proportional  to  its  own  strength.  So  that,  if  we  can  ascer- 
tain how  much  this  second  force  has  drawn  the  bullet  from  its 
path,  we  can  find  where  it  would  have  gone  if  the  second  force 
had  not  acted  on  it. 

It  has  been  found  by  experiment  that  if  a  bullet  (or  any 
other  object  heavy  enough  practically  to  neutralise  the  resist- 
ance of  the  atmosphere)  is  dropped  from  a  height,  the  attraction 
of  the  Earth  will  cause  it  to  fall  16  feet  in  a  second  of  time. 
And  we  have  seen  that  this  force  acts  on  the  bullet  even  when 
in  motion. 

Let  us,  then,  select  a  perfectly  level  piece  of  ground  (which 
does  not  share  in  the  convexity  of  the  Earth's  surface),  and  fire 
a  gun  horizontally  at  a  height  of  16  feet  above  the  ground. 
We  shall  find  that  the  bullet  (instead  of  keeping  on  in  a 
straight  line  parallel  with  the  ground)  will  bend  down  so  as  to 
strike  the  ground  in  exactly  one  second  of  time.  If  the  act  of 
firing  the  gun  is  made  to  release  another  bullet  at  the  same  in- 
stant and  from  the  same  elevation,  both  bullets  will  reach  the 
ground  at  the  same  instant,  one  of  them  some  distance  away, 
and  the  other  beneath  the  gun.  So  far  as  regards  the  time  in 
which  the  first-mentioned  bullet  reaches  the  ground,  it  will 
make  no  difference  whether  the  charge  of  powder  used  is  large 
or  small,  though  of  course  a  large  charge  will  make  the  bullet 
strike  farther  away  than  it  would  with  a  small  charge. 

Now  if  we  could  have  prevented  the  attraction  of  the  Earth 
from  pulling  the  flying  bullet  16  feet  out  of  its  course,  it  would 
have  kept  in  a  straight  line  at  the  same  distance  from  the 
ground. 

Thus  we  see  that  although  the  first  law  of  motion  cannot  be 
experimentally  proved  by  itself,  it  can  readily  be  tested  in  con- 
junction with  the  second  law.  The  only  difficulty  in  the  ex- 


166     HOW  TO   KNOW  THE   STARRY   HEAVENS 

periment  is  to  make  a  correct  allowance  for  the  atmospheric 
friction.  To  get  the  most  accurate  results,  the  experiment 
should  be  performed  in  a  vacuum,  but  this  is  easier  said  than 
done. 

THIRD  LAW  OF  MOTION 

The  Third  Law,  concerning  reaction,  is  true  of  both  attrac- 
tion and  repulsion.  It  is  not  only  true  that  a  magnet  will 
draw  a  piece  of  iron  to  it,  but  it  is  also  true  that  a  piece  of 
iron  will  draw  a  magnet  to  it.  When  a  gun  is  fired,  the  back- 
ward kick  of  the  gun  is  equal  to  the  forward  impulse  of  the 
bullet,  and  if  the  gun  and  bullet  were  of  the  same  weight  they 
would  both  go  the  same  distance,  but  in  opposite  directions. 

If  two  bullets  are  connected  by  a  string,  and  then  swung 
into  the  air,  they  will  circle  around  each  other  as  they  fly.  If 
both  are  of  the  same  weight,  they  will  swing  around  the  centre 
of  the  string.  If  one  is  heavier  than  the  other,  then  the  lighter 
one  will  make  the  larger  circles.  In  the  latter  case  the  influ- 
ence of  the  small  bullet  will  be  exerted  on  a  greater  mass,  and 
will  consequently  draw  it  less  out  of  its  course.  But  neverthe- 
less it  is  obvious  that,  as  in  the  case  of  the  bullet  fired  from  a 
gun,  the  action  and  reaction  will  be  equal.1 

It  is  rather  singular  that  Galileo,  while  discovering  and 
fighting  for  his  laws  of  motion,  entirely  ignored  Kepler's  dis- 
covery of  the  three  laws  of  planetary  motion,  dealt  with  in  the 
preceding  chapter.  This  has  been  charitably  attributed,  not 
to  vulgar  jealousy,  but  to  "  a  certain  unconscious  intellectual 
egotism,  not  always  unknown  to  the  greatest  minds."  —  Ency. 
Brit.  art.  "  Galileo." 

1  If  anyone  still  finds  these  laws  of  motion  hard  to  understand,  he  may  find 
his  difficulties  melt  away  as  he  reads  them  in  the  simple  form  in  which  they  were 
first  given  to  an  admiring  world.  They  were  published  as  follows : 

I.  Corpus  omne  perseverare  in  statu  quo  quiescendi  vel  movendi  uniformiter 
in  directum,  nisi  quatenus  illud  a  viribus  impressis  cogitur  statum  suum  mutare. 

II.  Mutationem  motus  proportionalem  esse  vi  motrici  impress®,  et  fiere  sectm- 
dum  lineam  rectam  qua  vis  ilia  imprimitur. 

III.  Actioni  contrariam  semper  et  aequalem  esse  reactionem ;  sive  corporum. 
duorum  actiones  in  se  mutuo  semper  esse  aequales  et  in  partes  contrarias  dirigi. 


CHAPTER  XV 

NEWTON'S  LAW  OF  GRAVITATION 

"  The  force  of  gravitation  acts  on  every  particle  of  matter,  and  hence  it  is  not 
confined  to  our  own  World.  By  its  action  the  heavenly  bodies  are  bound  to  one 
another,  and  thus  kept  in  their  orbits.  It  may  help  us  to  conceive  how  the  Earth 
is  supported,  if  we  imagine  the  Sun  letting  down  a  huge  cable,  and  every  star  in 
the  heavens  a  tiny  thread,  to  hold  our  globe  in  its  place.  ...  So  we  are  bound 
to  them,  and  they  to  us.  Thus  the  worlds  throughout  space  are  linked  together 
by  these  cords  of  mutual  attraction,  which,  interweaving  in  every  direction,  make 
the  Universe  a  unit."  —  J.  Dorman  Steele. 

THE  LAW  AND   ITS  PROOF 

THE  three  laws  of  motion  discussed  in  the  preceding  chapter 
were  at  first  confined  to  sublunary  dynamics.     But  Sir 
Isaac  Newton1  showed  that  the  heavenly  bodies  also  are  subject 
to  them.     He  theorised,  and  after  many  years  of  labour  proved, 
that  while  they  were  the  cause  of  the  peculiarities  described  in 
Kepler's  Laws,  they  were  themselves  the  result  of  a  general 
principle  known  as  the  Law  of  Gravitation. 
According  to  this  universal  law  — 

"  All  bodies  in  the  material  Universe  gravitate  toward  each  other 
with  a  force  which  is  directly  proportional  to  their  masses,  and  inversely 
proportional  to  the  squares  of  their  distances  from  one  another." 

Perhaps  the  simplest  form  in  which  this  law  can  be  correctly 
stated  is  that  "  every  particle  of  matter  in  the  Universe  pulls 
every  other  particle  toward  it  with  a  force  which  decreases  as 
the  square  of  the  distance  increases." 

When  the  law  of  gravitation  was  first  brought  forward  in  a 
theoretical  manner,  it  seemed  to  be  very  feasible,  but,  at  the 
same  time,  to  be  incapable  of  proof.  The  assumption  that  the 

1  Born  in  1642;  died  in  1727. 


168     HOW  TO   KNOW  THE   STARRY   HEAVENS 

attracting  force  varies  directly  in  proportion  to  the  masses 
hardly  required  any  proof ;  it  simply  meant  that  a  body  com- 
posed of  three  atoms  would  attract  three  times  as  strongly  as  a 
body  composed  of  one  atom.  It  could  no  more  be  questioned 
than  the  statement  that  three  equal  masses  of  iron  will,  under 
similar  conditions,  weigh  three  times  as  much  as  one  such  mass 
alone.  But  the  assumption  that  the  attraction  of  one  body  for 
another  varies  in  the  inverse  proportion  to  the  square  of  the 
distance  separating  them  was  neither  obvious  nor  easily  proved. 
There  was  this  to  be  said  in  favour  of  the  proposition,  however, 
that  light,  heat,  and  similar  forces  vary  in  intensity  according 
to  this  rule.  For  example,  if  a  small  lamp  shines  through  a 
square  twelve-inch  hole  in  a  screen  one  yard  away,  so  as  to 
light  up  part  of  another  screen  two  yards  from  the  lamp,  it  will 
be  found  that  the  area  illuminated  on  the  second  screen  contains 
four  square  feet.  It  is  therefore  four  times  as  large  as  the  hole 
in  the  first  one,  and  the  intensity  of  the  light  is  obviously 
reduced  to  one  quarter  of  what  it  was  at  half  the  distance.  If 
the  second  screen  be  moved  to  a  distance  of  three  yards  from 
the  lamp,  it  will  be  found  that  the  area  illuminated  measures 
nine  square  feet.  It  is  therefore  nine  times  as  large  as  the  hole 
in  the  first  screen,  and  the  intensity  is  evidently  only  one  ninth 
of  what  it  was  at  one  third  the  distance.  The  rule  holds  good 
for  any  distance,  provided  that  none  of  the  light  is  absorbed  by 
fog,  etc. 

Newton  theorised  that  the  attraction  of  all  the  atoms  com- 
posing the  Earth  would  act  as  though  the  pull  came  from  the 
centre  of  the  Earth.  Now  this  attraction  is  strong  enough  to 
cause  a  body  on  the  surface  (about  4,000  miles  from  the  centre  of 
attraction)  to  fall  16  feet  in  one  second  of  time.  If  this  law 
of  gravitation  is  true,  then  an  object  twice  as  far  from  the  centre 
of  the  Earth  should  drop  only  one  quarter  as  far  in  the  same 
interval  of  time,  an  object  three  times  as  far  from  the  centre 
should  fall  only  one  ninth  as  far  in  a  second,  and  so  on  for  all 
distances. 

Newton  could  easily  have  proved  or  disproved  this  theory  if 


FIG.   84.  —  MOUNT   LOWE  OBSERVATORY,   IN  SOUTHERN  CALIFORNIA 


NEWTON'S  LAW  OF  GRAVITATION          169 

he  could  have  gone  up  4,000  and  8,000  miles  to  see  how  far  the 
Earth's  attraction  would  cause  a  body  at  those  distances  to  fall 
in  a  second.  But  unfortunately  he  was  not  able  to  do  this. 

Finally  he  found  a  way  to  get  over  the  difficulty  by  taking 
advantage  of  the  presumed  fact  that  our  Moon  is  kept  in  her 
orbit  by  the  attraction  of  the  Earth.  It  has  been  already  shown 
that  on  the  Earth's  surface  a  bullet  fired  horizontally  from  a  gun 
is,  in  one  second,  pulled  16  feet  out  of  its  direct  course  by  the 
attraction  of  the  Earth  beneath.  That  is  to  say,  it  falls  just  as 
fast  when  it  is  flying  through  the  air  as  when  it  is  simply 
dropped  from  the  hand.  Now  the  Moon  is  60  times  as  far  from 
the  centre  of  the  Earth  as  we  are.  Let  us  look  on  it  as  a  big 
bullet  fired  horizontally  at  that  height.  We  shall  then  see  that 
the  Earth's  attraction  is  pulling  it  out  of  the  straight  course 
it  would  follow  if  suddenly  left  to  itself.  The  theory  is  that 
the  Earth's  attraction  decreases  as  the  square  of  the  distance 
increases.  The  square  of  60  is  3,600,  so  that  the  Moon  should, 
in  one  second  of  time,  be  pulled  out  of  its  straight  course  the 
3,600th  part  of  16  feet.  This  is  about  one  twentieth  part  of  an 
inch,  and  it  fits  in  exactly  with  the  observed  motion  of  the 
Moon  in  its  orbit.  The  theory  is  therefore  true,  so  far  as  the 
Earth  and  Moon  are  concerned.1 

The  law  has  since  been  applied  to  all  the  planets  revolving 
around  our  Sun.  The  satellites  of  Jupiter,  etc.,  are  found  to 

1  Although  this  law  of  gravitation  was  unknown  before  Newton,  its  existence 
and  universality  were  suspected  by  Galileo  and  hinted  at  in  his  "  Dialogo  dei 
Massimi  Sistemi."  In  one  place  he  says: 

"  Le  parti  della  Terra  hanno  tal  propensione  al  centre  di  essa,  che  quando  ella 
cangiasse  luogo,  le  dette  parti,  benche  lontane  dal  globo  nel  tempo  delle  muta- 
tioni  di  esso,  lo  seguirebbero  per  tutto ;  esempio  di  ci6  sia  il  seguito  perpetuo  delle 
Medicee,  ancorche  separate  continuamente  da  Giove.  L'istesso  si  deve  dire  della 
Luna,  obligata  a  seguir  la  Terra." 

Rather  freely  translated,  this  reads : 

"  The  different  parts  of  the  Earth  have  such  a  propensity  toward  its  centre 
that,  though  it  should  change  its  place,  the  said  parts,  although  far  from  the  globe 
in  the  time  of  its  change,  would  follow  it  everywhere.  An  example  of  this  is  the 
perpetual  following  of  the  satellites  of  Jupiter,  although  always  separate  from  the 
planet.  There  is  a  similar  instance  in  the  case  of  the  Moon,  obliged  to  follow 
the  Earth."  ' 


170     HOW   TO   KNOW   THE   STARRY   HEAVENS 

conform  to  it,  and  the  cometary  messengers  of  heaven  are  bound 
by  it.  Even  the  starry  systems  appear  to  circle  in  complete 
bondage  to  this  universal  law  of  gravitation. 

PLANETS  SECURELY  TETHERED 

We  now  come  to  two  questions  which  often  puzzle  those  who 
have  not  made  a  study  of  the  subject.  Why  does  not  a  planet, 
when  it  reaches  its  perihelion,  continue  to  approach  the  Sun  in 
a  closing  spiral  till  it  finally  falls  into  it  ?  And  why  does  it 
not,  when  at  its  aphelion,  continue  to  recede  in  an  opening 
spiral,  and  finally  go  off  into  the  outer  darkness  of  space  ?  So  far 
as  the  principles  involved  are  concerned)  these  questions  are  not 
difficult  to  answer  satisfactorily.  But  mathematics  are  neces- 
sary if  we  wish  to  go  into  details  and  prove  every  point. 

Let  us  take  a  single  planet  moving  in  a  very  eccentric  orbit. 
We  will  suppose  that  it  has  passed  its  aphelion  and  is  approach- 
ing that  part  of  its  orbit  where  it  will  be  nearest  to  the  Sun. 
As  its  distance  decreases,  the  increasing,  attraction  of  the  Sun 
(being  in  the  same  general  direction  as  that  in  which  the  planet 
is  moving)  gradually  increases  its  speed,  and  therefore  straightens 
its  path.  For,  the  deflection  Sunward  being  so  many  inches 
per  minute,  if  the  planet  moves  faster  its  course  will  be  straighter.1 
The  result  is  that  the  planet  sweeps  by  the  Sun  with  a  rush 
that  the  increased  attraction  cannot  check.2 

As  the  planet's  distance  from  the  Sun  increases,  the  decreasing 
attraction  (being  almost  opposite  to  the  direction  in  which  the 
planet  is  moving)  gradually  checks  its  speed.  It  therefore 
bends  in  toward  the  Sun  and  finally  begins  to  approach  again. 

Where  the  planet  moves  in  an  orbit  of  small  eccentricity,  the 

1  This  is  seen  on  our  Earth,  where  a  large  charge  of  powder  will  send  a  bullet 
swifter  and  therefore  straighter  than  a  small  charge.     If  we  could  stand  on  the 
summit  of  a  lofty  mountain,  and  fire  a  cannon  horizontally  with  just  sufficient 
charge,  the  projectile  would  sweep  around  the  Earth,  and  (possibly)  knock  in  the 
breech-plug  of  the  cannon  from  behind. 

2  A  similar  thing  takes  place  with  the  pendulum  of  a  clock.     The  Earth's  at- 
traction brings  it  down  on  one  side  with  a  rush  that  carries  it  up  on  the  other 
side. 


I    UNlv  .  votTY 

L 

NEWTON'S   LAW   OF  GRAVITATION          171 


above  phenomena  are  not  so  marked,  but  the  principle  .is 
the  same.  In  the  absence  of  a  resisting  medium  there  is  not 
the  slightest  possibility  of  a  planet  getting  into  a  closing  spiral. 
And  an  opening  one  is  only  possible  where  tidal  influences  are 
very  strong.  So  we  need  not  be  afraid  of  either  falling  into  the 
Sun  or  of  breaking  loose  and  drifting  into  the  outer  darkness. 
These  are  only  possibilities  of  the  remote  future,  long  after  the 
human  race  shall  have  passed  away  from  other  causes. 

Where  a  number  of  planets  are  revolving  around  the  same 
sun,  at  different  distances,  their  mutual  attractions  slightly 
interfere  with  their  orbits,  producing  certain  periodical  jper^wr- 
JataVws.  It  was  the  study  of  some  of  these  irregularities  which 
led  to  the  discovery  of  the  planet  Neptune,  1,000,000,000  miles 
beyond  Uranus,  which  had  previously  been  regarded  as  the  out- 
side planet  of  our  system. 

WHAT  IS  GRAVITATION? 

It  appears  certain  that  all  matter  is  under  the  absolute  con- 
trol of  the  attraction  of  gravitation.  Atoms  and  suns  are  alike 
ruled  by  this  law.  Every  motion  is  regulated  by  it,  every  non- 
motion  is  the  result  of  it. 

Although  gravitation  is  now  thoroughly  understood,  both  as 
to  its  mode  of  action  and  the  measure  of  its  power,  there  is  yet 
some  uncertainty  as  to  its  nature.  It  is  generally  regarded  as  a 
universal  property  inherent  in  matter  itself,  but  some  think  that 
it  may  be  an  independent  energy  controlling  matter  from  the 
outside. 

The  original  idea  was  that  gravitation  reaches  out  from  one 
particle  to  another,  even  when  they  are  separated  by  great  dis- 
tances, with  nothing  to  connect  them,  —  in  fact  that  there  is 
action  at  a  distance  without  a  medium.  This  idea  has  had  to 
be  abandoned,  for  it  is  evident  that  a  thing  cannot  act  where  it 
is  not  present.  So  a  connecting  ether  has  been  "  invented  "  to 
carry  the  energy  of  gravitation  (and  what  is  known  as  radiant 
energy)  across  from  one  particle  of  matter  to  another.  Whether 


172    HOW  TO   KNOW  THE   STARRY   HEAVENS 

this  ether  really  exists,  what  it  is  like,  and  how  it  acts,  are 
questions  that  still  keep  scientists  busy,  and  will  probably  not 
be  settled  for  some  time  to  come. 

EFFECTS  OF  GRAVITATION 

Whatever  it  is,  the  action  of  gravitation  is  always  mutual, 
and  appears  to  be  instantaneous.  It  operates  at  all  distances 
and  through  all  substances. 

Its  tendency,  suddenly  applied  to  scattered  bodies  previously 
at  rest,  would  be  to  bring  together  all  the  suns  and  worlds  of 
which  the  Universe  is  composed.  But,  if  applied  to  bodies  al- 
ready in  motion,  its  effect  would  be  to  change  direct  motions 
into  curved  orbits  like  those  of  the  planets. 

Its  effect  on  any  isolated  body  (like  the  Sun,  or  our  Earth) 
is  to  cause  its  particles  to  arrange  themselves  in  the  form  of  a 
sphere,  more  or  less  flattened  when  there  is  any  considerable 
rotation.  In  this  sphere  the  densest  forms  of  matter  naturally 
gravitate  toward  the  centre,  and,  while  "the  globe  possesses  any 
considerable  amount  of  internal  heat,  there  may  be  an  outer 
layer  of  liquid,  with  expansive  gases  outside  of  all. 

On  the  surface  of  such  isolated  bodies  its  effects  are  almost 
inappreciable,  the  only  noticeable  effect  being  that  every  par- 
ticle of  matter  clings  to  the  planet  with  an  intensity  varying 
directly  with  the  mass  of  the  planet,  and  inversely  with  the 
square  of  the  distance  between  the  body  and  the  centre  of  the 
planet.  On  the  same  globe  all  things  on  the  surface  are  at  prac- 
tically the  same  distance  from  the  centre.  Therefore  they  are  all, 
whether  heavy  or  light,  attracted  with  the  same  intensity,  and 
(in  a  vacuum)  fall  with  the  same  velocity.  The  weight  of  an 
object  equals  its  mass  multiplied  by  the  force  of  gravity.  If  it 
were  removed  to  a  planet  where  the  force  of  gravity  was  twice  as 
great,  its  weight  would  be  doubled,  though  its  mass  would  re- 
main the  same. 


FIG.  85.  —  SPIRAL  NEBULA  IN   URSA  MAJOR  (M  81) 
Lick  photograph. 


NEWTON'S   LAW   OF   GRAVITATION  173 


INTENSITY  OF  GRAVITATION 

As  regards  the  strength  of  the  attraction  of  gravitation,  it  is 
really  very  small,  being  more  than  a  million  times  weaker  than 
that  of  magnetism.  Two  masses,  each  of  465,000  American 
tons,  attract  each  other  with  a  force  of  one  pound  when  they 
are  a  mile  apart.  If  the  distance  between  them  be  doubled, 
the  attraction  is  reduced  to  four  ounces.  If  the  distance  be 
reduced  to  half  a  mile,  it  is  increased  to  four  pounds.  If  one 
of  the  masses  be  doubled  in  size  and  weight  the  attractive  force 
is  doubled,  while  if  both  masses  be  doubled,  the  attraction  is 
quadrupled. 

OMNIPOTENCE  OF  GRAVITATION 

The  above  example,  taken  by  itself,  might  lead  one  to  suppose 
that  the  attraction  of  gravitation  is  of  very  slight  importance  in 
a  Universe  where  the  distances  separating  the  suns  and  worlds 
are  so  vast.  But  other  forces  are  local,  or  operate  only  under 
certain  conditions,  while  gravitation  appears  to  be  universal  and 
to  act  under  all  conditions.  It  therefore  becomes  an  all-control- 
ling force  whose  immensity  the  mind  of  man  cannot  realise.  It 
possesses  some  of  the  attributes  of  Deity,  being  omnipotent, 
omnipresent,  and  eternal.  It  controls  the  infinitely  great  and 
the  infinitely  small.  It  regulates  the  movements  of  every  speck 
of  dust  that  dances  in  a  sunbeam,  and,  at  the  same  time,  of 
every  sun  that  sweeps  through  the  boundless  depths  of  space. 

The  following  illustration  will  give  some  faint  idea  of  the  im- 
mense power  of  the  attraction  of  gravitation  : 

In  going  around  the  Sun,  the  Earth  travels  about  19  miles  in 
a  second  of  time.  During  the  same  interval  the  Sun  pulls  the 
Earth  toward  him  about  one  eighth  of  an  inch.  The  power 
exerted  to  produce  this  insignificant  change  of  direction  is  so 
enormous  that  if  the  solar  gravitation  were  to  be  replaced  by 
steel  telegraph  wires  they  would  have  to  be  attached  to  the 
entire  Sunward  side  of  the  Earth,  considerably  closer  together 
than  the  stalks  in  a  flourishing  wheat  field.  If  the  bond  con- 


174     HOW  TO   KNOW  THE   STARRY   HEAVENS 

sisted  of  a  single  cast-iron  rod,  it  would  have  to  be  thicker  than 
the  Earth  itself  in  order  to  stand  the  strain.  Yet  the  mutual 
attraction  of  the  two  bodies  keeps  them  together  without  any 
visible  bond. 

Professor  Duffield  says  of  this  law  : 

"  We  cannot  but  regard  it  as  the  most  important  truth  in  the 
whole  book  of  Nature,  and  its  discovery  as  the  most  interesting  event 
in  the  history  of  physical  science.  As  there  is  but  one  material 
Universe,  and  the  law  of  gravitation  solves  the  enigma  of  its  struc- 
ture, no  other  problem  of  equal  interest  and  importance  can  ever 
occupy  the  attention  of  the  student  of  Nature." 

The  above  quotation,  while  paying  a  just  tribute  to  Newton's 
discovery,  really  overrates  the  scope  and  importance  of  the  law 
of  gravitation.  As  Ernst  Haeckel  says  : 

"  Newton  had  the  immortal  merit  of  establishing  the  law  of  gravi- 
tation and  embodying  it  in  an  indisputable  mathematical  formula. 
Yet  this  dead  mathematical  formula,  on"  which  most  scientists  lay 
great  stress,  as  so  frequently  happens,  gives  us  merely  the  quantitative 
demonstration  of  the  theory  ;  it  gives  us  no  insight  whatever  into  the 
qualitative  nature  of  the  phenomena.  The  action  at  a  distance  with- 
out a  medium,  which  Newton  deduced  from  his  law  of  gravitation, 
and  which  became  one  of  the  most  serious  and  most  dangerous 
dogmas  of  later  physics,  does  not  afford  the  slightest  explanation  of  the 
real  causes  of  attraction  ;  indeed  it  long  obstructed  our  way  to  the 
real  discovery  of  them." 

The  fact  is  that  we  are  yet  groping  almost  in  the  dark,  and 
need  a  second  Newton  to  tell  us  why  the  law  of  gravitation 
acts  as  he  proved  it  to  act. 

PRECESSION  AND  NUTATION 

In  Chapter  XII  the  slow  reeling  of  the  Earth's  axis  was 
described,  without  any  explanation  except  the  remark  that  it 
was  allied  to  the  wabbling  of  a  child's  spinning-top.  As  the 
phenomenon  is  one  of  the  results  of  the  law  of  gravitation,  and 


NEWTON'S   LAW  OF  GRAVITATION          175 

was  first  explained  by  Newton,  a  few  words  concerning  its 
cause  may  not  be  out  of  place  here.  It  should  be  remembered 
that  the  intensity  of  the  Sun's  attraction  on  a  planet  varies  with 
the  square  of  the  distance.  And  it  should  not  be  forgotten 
that  it  acts  independently  on  each  and  every  individual  atom 
composing  the  planet.  An  atom  at  the  planet's  centre  is 
attracted  less  than  an  atom  on  the  surface  facing  the  Sun,  and 
is  attracted  more  than  an  atom  on  the  surface  away  from  the 
Sun.  The  difference  in  intensity  produces  the  same  result  as 
though  the  nearest  atom  was  attracted,  and  the  farthest  atom 
was  repelled.  In  a  perfectly  spherical  planet  these  differences 
would  neutralise  one  another,  and  the  effect  would  be  the  same 
as  though  all  the  atoms  were  at  the  centre  of  the  planet. 
When  the  planet's  equator  is  bulged  out  by  rotation,  the  effect 
is  the  same,  providing  that  the  equator  of  the  planet  coincides 
with  the  plane  of  its  orbit.  But  when  the  planet's  equator  is 
tipped  up  at  an  angle  with  the  p]ane  of  the  orbit  (as  is  the 
case  with  our  Earth),  the  relative  attraction  and  repulsion,  on 
the  equatorial  bulging,  tend  to  reduce  the  angle  at  which  the 
equator  is  tipped.  The  rotation  of  the  planet,  however,  modi- 
fies the  movement,  so  that  (in  summer  and  winter)  an  atom 
on  the  surface,  at  the  equator,  makes  a  lower  curve  and 
crosses  the  ecliptic  a  trifle  behind  the  place  where  it  would 
otherwise  have  crossed.  This  of  course  has  the  effect  of  mak- 
ing the  polar  axis  reel  slowly  back,  as  already  described.  The 
effect  is  trifling,  but  cumulative,  so  that  in  the  course  of  time 
the  axis  wabbles  completely  round.  In  the  case  of  our  Earth 
the  process  is  greatly  helped  by  the  attraction  of  the  Moon,  yet 
it  is  so  slight  that  it  takes  26,000  years  for  the  poles  to  make 
one  wabble.  At  the  equinoxes  the  action  ceases,  so  that  the 
circles  described  by  the  polar  axis  have  a  tremulous  or  wavy 
outline.  As  the  Moon's  orbit  is  a  little  inclined  to  the  Ecliptic 
and  has  a  "  precession  "  of  its  own,  another  irregularity  is  pro- 
duced every  19  years.  These  two  irregularities  in  the  Earth's 
precession  are  together  known  by  the  name  of  nutation. 


176     HOW   TO   KNOW  THE   STARRY  HEAVENS 


CAUSE    OF  REPULSION 

At  first  sight,  the  theory  of  gravitation  does  not  seem  to  ex- 
plain such  phenomena  as  the  solar  flames  and  corona.  These, 
and  especially  the  latter,  are  evidently  acted  upon  by  a  repellent 
force  of  enormous  intensity.  The  explanation  seems  to  be  that 
radiant  light  tends  to  push  away  any  substance  that  it  strikes. 
In  large  bodies  this  repulsion  is  very  small  when  compared  with 
the  force  of  gravitation.  But  the  former  acts  according  to  the 
surface,  and  the  latter  to  the  mass.  As  bodies  decrease  in  size, 
the  repellent  force  of  light  decreases  by  the  square,  while  the 
force  of  gravitation  decreases  by  the  cube.  On  extremely  small 
particles  gravitation  is  overpowered  by  a  continuous  repulsion 
two,  ten,  or  twenty  times  as  great,  according  to  their  size.  They 
are  therefore  driven  off  like  a  bullet  out  of  a  rifle,  only  hundreds 
of  times  more  rapidly. 

Corona.  —  In  the  case  of  the  corona,  the  eruptive  particles 
are  so  minute  that  they  appear  to  be  driven  clear  away  into 
interstellar  space.  On  the  way,  some  of  them  are  intercepted 
by  the  planets.  As  they  are  charged  with  negative  electricity, 
and  the  planets  are  huge  "electrical  machines,"  they  arrange 
themselves  according  to  the  lines  of  force,  and  produce  such 
electrical  phenomena  as  the  Zodiacal  Light,  the  Gegenschein, 
the  Aurora  Borealis,  etc.  All  the  luminous  stars  send  off  simi- 
lar negatively  electrified  particles,  and  when  these,  in  their 
travels,  come  across  a  large  mass  of  uncompressed  matter,  they 
cause  its  surface  to  glow  like  the  rarefied  gas  in  a  vacuum  tube. 
This  appears  to  be  the  reason  why  nebulae  are  luminous,  though 
cold  with  the  fearful  cold  of  interstellar  space. 

Solar  Flames.  —  In  the  case  of  the  solar  flames,  the  eruptive 
particles  are  probably  larger,  and  therefore  come  to  a  standstill 
sooner  or  later.  They  are  then  buoyed  up,  not  by  an  atmos- 
phere, as  our  clouds  are,  but  by  the  pressure  of  light  radiating 
from  below.  If  they  join  together  to  form  larger  particles,  they 
fall  back  to  the  surface  of  the  Sun,  like  the  raindrops  from  our 
watery  clouds. 


NEWTON'S  LAW  OF  GRAVITATION        177 


A   WREATH  OF  SMOKE 

Sometimes,  on  a  quiet  evening,  just  before  sundown,  when 
hardly  a  breath  of  air  was  stirring,  I  have  watched  the  blue 
rings  and  spirals  of  tobacco-smoke,  slowly  curling,  twining,  and 
eddying  in  the  level  glints  of  dying  sunshine. 

Now  if  you  will  imagine  that  the  atoms  of  carbon  in  that 
wreath  of  tobacco-smoke  are  mighty  suns  in  every  stage  of  solar 
life,  —  from  the  spiral  nebulae  of  solar  infancy  to  the  dark,  cold, 
and  lifeless  wrecks  of  superannuated  suns,  —  then  the  curling 
and  eddying  smoke  will  represent  the  Universe. 

For  the  great  telescopes  of  our  observatories  show  us  star- 
clusters  and  nebulae  extended  through  space  in  gigantic  rings, 
eddies,  and  spirals.  If  we  could  watch  this  celestial  cloud  of 
smoke  for  a  few  millions  of  years,  it  is  almost  certain  that  we 
should  see  these  curves  and  spirals  change  from  form  to  form 
like  wreaths  of  smoke.  Through  all  eternity  the  star-dust  of 
which  the  Universe  is  composed  is  eddying  and  circling  through 
space  that  has  no  limits. 

And  the  very  same  laws  which  regulate  the  curling  of  the  blue 
tobacco-smoke  regulate  the  eddying  and  circling  of  the  innu- 
merable nebulae,  suns,  and  worlds  which  compose  our  mighty 
COSMOS. 

NOTE.  The  laws  briefly  dealt  with  in  the  above  three  chapters  are  more  fully 
discussed  and  illustrated  in  Sir  George  Airy's  "  Popular  Astronomy,"  which  was 
written  for  non-mathematical  readers;  in  George  C.  Comstock's  more  recent 
"  Textbook  of  Astronomy ;  "  and  in  many  other  works. 


12 


CHAPTER  XVI 

ANCIENT  COSMOGONIES,  AND  THE  NEBULAR 
HYPOTHESIS 

"  In  the  intellectual  infancy  of  a  savage  state,  Man  transfers  to  Nature  his 
conceptions  of  himself,  and,  considering  that  everything  he  does  is  determined 
by  his  own  pleasure,  regards  all  passing  events  as  depending  on  the  arbitrary 
volition  of  a  superior  but  invisible  power.  .  .  .  After  Reason,  aided  by  Experi- 
ence, has  led  him  forth  from  these  delusions  as  respects  surrounding  things,  he 
still  clings  to  his  original  ideas  as  respects  objects  far  removed.  ...  But  as 
reason  led  him  forth  from  fetishism,  so  in  due  time  it  again  leads  him  forth  from 
star-worship.  .  .  .  Philosophically  speaking,  he  is  exchanging,  by  ascending 
degrees,  his  primitive  doctrine  of  arbitrary  volition  for  the  doctrine  of  law." 
—  Dr.  J.  W.  Draper. 

FACTS  VERSUS  THEORIES 

THE  human  race  is  not  old  enough  to  have  watched  the 
development  of  suns  and  worlds  to  any  appreciable 
extent.  We  cannot  therefore  know  for  an  absolute  certainty 
that  such  a  process  is  at  work  throughout  the  Universe.  Yet 
thinking  men  are  naturally  led,  by  their  earthly  experiences 
and  celestial  observations,  to  believe  in  such  a  development, 
and  to  theorise  as  to  how  the  Universe  has  reached  its  present 
condition.  They  have  even  gone  beyond  the  present,  and 
speculated  as  to  the  changes  which  it  will  undergo  in  the 
future. 

Even  a  false  theory  has  its  value  as  a  stepping-stone  to  lead 
to  a  true  one.  It  is  well,  therefore,  to  theorise  even  where  we 
have  no  direct  method  of  proving  the  truth  or  falsity  of  our 
speculations.  When  a  theory  gives  a  reasonable  explanation  of 
observed  phenomena,  it  must  have  some  truth  in  it.  And 
when  it  not  only  explains  all  known  phenomena,  but  also 
enables  us  to  discover  and  explain  fresh  ones,  we  may  regard  it 


COSMOGONIES,   AND  NEBULAR   HYPOTHESIS     179 

as  a  valuable  help  for  the  upbuilding  of  the  Temple  of 
Knowledge. 

Still  it  is  well  for  us  to  remember  that  an  unproved  theory 
is  not  necessarily  a  permanent  part  of  that  Temple  of  Knowl- 
edge. It  may  be  only  a  piece  of  scaffolding  that  will  have  to 
come  down  when  the  building  is  further  advanced. 

One  of  the  great  troubles  in  the  past  has  been  the  tendency 
to  mistake  unproved  theories  for  known  facts,  and  to  stretch, 
twist,  or  ignore  all  facts  which  refuse  to  fit  in  with  them. 
This  is  evidently  a  serious  mistake.  As  Sir  William  Crookes 
said  at  Berlin,  a  short  time  ago : 

"  It  must  never  be  forgotten  that  theories  are  only  useful  as  long  as 
they  admit  of  the  harmonious  correlation  of  facts  into  a  reasonable 
system.  Directly  a  fact  refuses  to  be  pigeon-holed,  and  will  not  be 
explained  on  theoretic  grounds,  the  theory  must  go,  or  it  must  be 
revised  to  admit  the  new  fact." 

With  this  necessary  warning  I  will  now  proceed  to  outline 
a  few  of  the  theories  which  have  been  held  with  regard  to  the 
past,  present,  and  future  history  of  the  Universe. 

HISTORICAL   SPECULATIONS 

There  are  in  existence  several  different  classes  of  what  pro- 
fess to  be  histories  of  the  Universe  in  general  and  of  our  World 
in  particular.  Some  of  them  have  come  down  to  us  from  pre- 
historic times,  while  others  are  quite  modern.  They  differ  so 
much  from  one  another  that  it  is  evident  they  cannot  all  be 
historical.  If  one  is  true,  the  others  must  be  more  or  less 
fictitious. 

Some  of  the  more  ancient  of  the  "  historians  "  professed  to 
start  from  the  very  beginning  of  things,  although  they  found  it 
a  difficult  thing  to  do.  They  began  either  with  a  primeval 
Chaos  that  always  existed,  or  with  a  world-egg  that  no  hen 
ever  laid.  Out  of  one  of  these  they  evolved  the  Earth  and 
all  that  is  therein,  along  with  such  trifles  as  the  Sun,  Moon, 
planets,  and  stars. 


180     HOW  TO   KNOW  THE   STARRY  HEAVENS 

The  more  modern  ones  do  not  profess  to  know  of  any  original 
Chaos,  and  they  doubt  the  veracity  of  the  primitive-egg  story. 
They  modestly  content  themselves  with  stating  what  they  be- 
lieve to  have  been  the  course  of  events  since  a  certain  time, 
beyond  which  they  admit  that  they  have  no  direct  knowledge. 
And  as  far  as  possible  they  adopt  those  theories  which  appear 
to  be  supported  by  evidence,  and  to  be  in  harmony  with  the 
laws  of  Nature  as  we  know  them  here  and  now. 

These  different  "histories"  may  be  more  or  less  definitely 
divided  into  the  following  four  classses. 
I.   Primitive  Creationism. 
II.   Pseudo  Creationism. 

III.  Evolutionary  Creationism. 

IV.  Modern  Evolutionism. 

I  must  here  content  myself  with  giving  a  very  brief  sketch 
of  the  leading  varieties  of  these  classes,  beginning  with  the 
various  forms  of  Creationism,  which  I  have  divided  into  Primary, 
Secondary,  and  Tertiary. 

I.   PRIMITIVE  CREATIONISM   (PRIMARY  STAGE) 

In  early  times,  when  unenlightened  men  first  began  to 
speculate  as  to  the  origin  and  history  of  the  World  and  its 
surroundings,  they  found  no  one  to  tell  them  where  the  Earth 
had  come  from  or  how  it  had  come  into  being. 

The  wise  men  of  each  tribe  naturally  had  to  answer  many 
inquiries  on  the  subject.  If  they  had  frankly  admitted  that 
they  did  not  know  the  origin  and  destiny  of  all  things,  they 
would  have  lost  their  reputation  for  wisdom.  So  it  was  abso- 
lutely necessary  for  them  to  invent  some  story  which,  if  it  did 
not  satisfy  the  mind  of  a  child,  would  at  least  put  a  stop  to  its 
questions.  This  is  probably  the  way  in  which  all  creation 
stories  have  originated,  fresh  details  being  gradually,  and  often 
unconsciously,  added  by  successive  generations. 

From  their  own  limited  observations  and  experiences,  these 
primitive  men  naturally  inferred  that  the  World  could  not 
always  have  existed  as  they  found  it.  They  concluded  that  it 


COSMOGONIES,  AND  NEBULAR  HYPOTHESIS     181 

must  —  some  time,  some  where,  and  in  some  manner  —  have 
been  either  hatched  or  born.  The  only  other  explanation  they 
could  think  of  was  that  it  might  have  been  made  by  the  Father 
of  the  Gods,  for  his  own  amusement. 

As  a  rule  the  ancients  were  entirely  ignorant  of  all  except 
their  immediate  surroundings.  This  ignorance  compelled  them 
to  view  all  things  from  a  flat-world  standpoint,  which  natu- 
rally and  necessarily  decided  the  character  of  their  specula- 
tions on  the  origin  of  things  in  general.  Having  no  one  to 
tell  them  the  facts  of  the  case,  they  gradually  evolved  from 
their  own  inner  consciousness  a  number  of  mythical  his- 
tories whose  ephemeral  nature  fitted  in  with  their  insignificant 
Cosmos. 

The  best  known  of  these,  so  far  as  our  part  of  the  World 
is  concerned,  is  an  old  Semitic  legend,  the  modern  form  of 
which  has  been  poetically  narrated  by  Milton  in  his  "Paradise 
Lost." 

According  to  this  account  the  World  (including  its  attendant 
Sun,  Moon,  and  stars)  is  a  manufactured  contrivance,  invented 
and  constructed  by  one  who  is  known  as  the  Architect,  or 
Creator,  of  the  Universe.  Although  the  account  professes  to 
start  from  the  beginning,  it  leaves  the  origin  of  the  Creator 
himself  in  the  primeval  darkness,  and  even  intimates  that  he 
had  no  origin,  but  always  existed. 

This  Architect  of  the  Universe  is  said  to  have  "  created  "  the 
World  and  its  appurtenances.  This  prodigious  work  having 
been  accomplished  in  six  days,  the  Creator,  we  are  told,  "  rested 
on  the  seventh  day,  and  was  refreshed."  He  now  keeps  it  in 
existence  and  repair,  and  personally  superintends  everything 
that  takes  place  on  it. 

Owing  to  the  machinations  of  an  ambitious  and  unscrupu- 
lous servant,  whom  he  had  himself  brought  into  existence, 
things  did  not  run  smoothly  in  the  newly  created  World.  Its 
history  (past,  present,  and  future)  embraces  six  thousand  years 
of  strife  and  one  millennium  of  peace,  corresponding  to  the  six 
days  of  work  and  one  of  rest.  At  the  close  of  the  seven  thou- 


182     HOW   TO   KNOW   THE   STARRY   HEAVENS 

sand  years  the  World  and  its  Sun,  Moon,  and  stars  will  be 
destroyed,  and  will  be  succeeded  by  "  a  new  Heaven  and  a 
new  Earth,"  to  endure  for  ever. 

II.    PSEUDO  CREATIONISM  (SECONDARY   STAGE) 

The  above  is  only  one  out  of  a  number  of  primitive  creation 
stories.  In  the  course  of  time  these  childlike  "  histories " 
became  looked  upon  as  not  only  true  but  also  "  inspired."  It 
was  actually  made  a  punishable  offence  to  doubt  their  truth 
and  inspiration.  So,  when  men  came  to  know  something  as  to 
the  actual  dimensions  of  the  Universe,  and  the  relative  im- 
portance of  the  Earth  in  that  Universe,  they  recognised  the 
improbable  nature  of  these  stories,  yet  hesitated  about  rejecting 
them  as  untrue  and  uninspired.  The  accounts  were  therefore 
"  spiritually  interpreted,"  to  fit  in  with  the  new  order  of  things. 
In  some  cases  considerable  ingenuity  has  been  used  to  accom- 
plish this.  For  example,  when  Astronomy  had  to  be  accepted, 
the  seven  thousand  years  of  strife  and-  peace  were  declared  to 
refer  to  our  World  alone.  When  Geology  had  to  be  accepted, 
the  seven  days  of  creation  and  rest  were  "  spiritualised  "  into 
meaning  seven  immense  periods  of  time,  long  enough  for  suns 
and  worlds  to  develop  out  of  "fire-mist."  The  inconvenient 
fact  that  the  Sun  and  stars  were  treated  as  mere  appendages 
of  the  Earth,  was  explained  as  a  "  pious  fraud  "  on  the  part  of 
the  inspired  writer,  rendered  necessary  by  the  ignorance  of  his 
readers. 

Some  people  still  accept  these  stories  as  true  history.  Others 
have  forced  themselves  to  the  curious  conclusion  that  they  are 
not  true  historically,  but  that  they  are  allegories,  containing 
spiritual  truths,  perceptible  only  to  those  who  are  spiritually 
minded.  All  intelligent  and  well-informed  people  now  know 
that  these  accounts  are  simply  primitive  speculations  about 
the  unknown  past.  But  many  of  them  refrain  from  hurting  the 
feelings  of  others  by  openly  saying  so. 


COSMOGONIES,  AND  NEBULAR  HYPOTHESIS     183 

in.     EVOLUTIONARY  CREATIONISM  (TERTIARY  STAGE) 

Among  those  who  have  rejected  Oreationism  in  both  its 
primary  and  secondary  forms,  there  are  many  who  still  adhere 
to  it  in  an  attenuated  tertiary  form.  For  want  of  time  or  lack 
of  inclination  to  formulate  a  better  and  truer  theory,  they  take 
it  for  granted  that  in  a  very  remote  past  an  Infinite  and  Eternal 
Being  created  an  all-pervading  substance,  or  matter  out  of 
nothing,  and  endowed  it  with  certain  permanent  qualities  and 
energies.  He  then  allowed  it  to  evolve  without  any  further 
interference.  The  resulting  physical  and  chemical  movements 
of  this  matter  have  since  produced  the  Universe  as  we  know  it. 

Some,  however,  think  that  at  one  stage  in  each  world  there 
was  a  second  creative  act,  —  the  introduction  of  organic  life  in 
its  lowest  form.  From  this  primitive  single-celled  organism 
all  the  various  forms  of  animal  and  vegetable  life  have  since 
developed  by  natural  laws. 

Another  interference  with  mundane  events  is  believed  in  by 
some,  —  the  implanting  of  an  immortal  soul  in  man  after  his 
body  had  sufficiently  developed  by  the  operation  of  natural 
laws.  This  immortal  soul  was  then  left  to  work  out  its  own 
salvation  or  condemnation,  without  interference. 

This  tertiary  and  evolutionary  creationism  is  upheld,  in  one 
or  other  of  its  forms,  by  a  more  intelligent  class  of  people  than 
those  who  are  still  in  the  primary  and  secondary  stages,  but  it 
appears  to  share  with  those  beliefs  the  disadvantage  of  not 
being  either  reasonable  or  true. 

However,  the  truth  itself  is  so  astounding  to  finite  creatures 
like  us,  and  long-inherited  prejudices  are  so  strong,  that  the 
majority  of  people  are  not  yet  able  to  grasp  or  ready  to  receive 
it.  In  the  meantime  this  evolutionary  creation-story  serves 
very  well  for  a  temporary  basis  on  which  to  build  a  truer  and 
more  reasonable  history  of  the  Universe.  It  will  be  thrown 
away  as  soon  as  it  has  outgrown  its  usefulness,  and  the  super- 
structure can  then  be  fitted  on  to  the  eternal  foundation  of 
actual  fact. 


184     HOW   TO   KNOW   THE   STARRY   HEAVENS 


THE  ORIGINAL  NEBULAR  THEORY 

On  the  basis  of  a  single  creative  act,  Kant,  Laplace,  and 
Herschel  I.  founded  their  celebrated  Nebular  Theory,  which 
first  attracted  notice  about  the  end  of  the  eighteenth  century.1 
It  is  rather  remarkable  that  these  three  great  men  arrived  at 
practically  the  same  conclusions  by  independent  and  entirely 
different  routes. 


FIG.  86.  —  ORIGINAL,  NKBULA,  AFTER  ITS  ROTATION 
HAS  PRODUCED  A  DISC-LIKE  FORM 


Immanuel  Kant2  was  led  to  it  in  his  youth  by  abstract 
philosophical  speculations.  He  outlined  the  theory  in  a  work 
which  was  published  in  1755,  but  as  it  did  not  excite  any 
interest,  he  turned  his  attention  to  other  speculations. 

Pierre  Laplace  3  was  led  to  its  consideration  by  mathematical 

1  Kepler  and  Tycho  Brahe  had  previously  speculated  that  the  Sun  and  stars 
were  condensations  from  celestial  vapours. 

2  Born  in  1724;  died  in  1804. 
8  Born  in  1744;  died  in  1827. 


COSMOGONIES,  AND  NEBULAR  HYPOTHESIS     185 

reasoning,  in  middle  age,  and  published  it  in  a  modest  footnote 
in  his  "  Systeme  du  Monde,"  1796.1 

Sir  William  Herschel2  was  led  to  it  by  his  lifelong  tele- 
scopic observations.  He  discussed  it  in  his  papers  to  the  Koyal 
Society. 

The  assumption  was  that  "  in  the  beginning  "  an  inconceiv- 
ably vast  and  attenuated  mass  of  intensely  heated  gas  was 


FIG.  87.  —  NEBULA  WITH  OUTER  RING,  LEFT  BEHIND  BY  CON- 
TRACTION AND  CONSEQUENT  QUICKENING  OF  ROTATION 

created  and  put  in  motion.  This  original  motion  led  —  with- 
out any  further  interference  —  to  its  gradual  condensation  into 
a  number  of  rotating  lens-shaped  nebulae  of  thin  gas.  One  of 
these  has  since  shrunk  and  developed  into  our  Solar  System, 
others  have  condensed  and  developed  into  the  stellar  systems 
which  surround  us,  and  thousands  (having  for  some  reason 
failed  to  develop)  still  exist  as  glowing  gaseous  nebulae.  They 

1  He  suggested  it  cautiously,  "  avec  la  defiance  que  doit  inspirer  tout  ce  qui 
n'est  poiut  un  resultat  de  1'observation  ou  du  calcul." 

2  Born  in  1738;  died  in  1822. 


186     HOW  TO   KNOW   THE   STARRY   HEAVENS 

all  move  according  to  the  natural  laws  originally  imparted  to 
matter  by  the  Creator,  and  eventually  discovered  by  Kepler, 
Galileo,  and  Newton. 

As  one  of  these  primary  rotating  nebulae  (subject  to  the 
mutual  attraction  of  its  own  particles)  gradually  condenses  into 
a  spheroidal  form,  it  naturally  increases  in  density  and  speed 


Fio.  88.  —  CENTRAL  CONDENSATION  SURROUNDED  BY  RINGS 

of  rotation.     Much  of  its  energy  is  also  turned  into  heat,  which 
radiates  into  space. 

The  tendency  of  every  moving  body  to  continue  in  a  straight 
line  (First  Law  of  Motion)  produces  a  centrifugal  force  which 
increases  with  the  increasing  speed  of  rotation.  At  last  the 
equatorial  part  of  the  spheroid  breaks  away  in  a  ring,  which 
may  continue  to  rotate  around  the  shrinking  central  body. 
Generally,  however,  it  breaks  up  into  a  secondary  rotating 
spheroid,  or  planet,  which  continues  to  revolve  around  the 
primary  one,  or  sun.  Each  planet  goes  through  the  same 
process  of  condensation,  throwing  off  similar  rings,  which 


COSMOGONIES,  AND  NEBULAR  HYPOTHESIS     187 

generally  collapse  into  tributary  satellites  or  moons  (see  Fig- 
ures 86  to  90). 

In  all  these  secondary  and  tertiary  bodies  the  original  heat 
is  gradually  dissipated  by  radiation  into  outer  space.  The  later 
rings  are  smaller  than  the  earlier  ones,  and  give  rise  to  smaller 
planets  and  moons,  which  go  through  the  various  stages  more 
rapidly  than  the  larger  ones. 

In  time  the  formation  of  rings  ceases,  but  the  central  spheroid 
continues  to  decrease  in  size  and  increase  in  density.  Although 


FIG.  89.  —  RINGS  COLLAPSING  INTO  PLANETS,  AND  CENTRAL 
CONDENSATION  TURNING  TO  A  LUMINOUS  SUN 

still  gaseous,  it  has  long  ceased  to  be  a  glowing  transparent 
nebula,  and  is  a  compact  bluish-white  sun,  radiating  an  im- 
mense amount  of  light  and  heat  into  space. 

Later  on,  its  colour  changes  to  a  yellowish  white,  and  after- 
ward to  yellow.  The  radiation  of  heat  into  outer  space  goes 
on  continuously,  so  that  in  time  it  cools  off  to  a  reddish  tinge. 
The  colour  deepens  to  crimson  and  gradually  fades  away.  The 
star  liquefies  and  then  becomes  solid.  The  crust  no  longer 


188     HOW  TO   KNOW  THE   STARRY   HEAVENS 

glows  with  light  and  heat,  so  that  the  star  ceases  to  be  visible 
from  outer  space. 

Meanwhile  the  various  worlds  to  which  it  has  given  rise 
have  cooled  off,  become  liquid,  crusted  over,  and  finally  solid- 
ified. Life  has  made  its  appearance  on  their  surfaces,  developed, 
flourished,  and  died  away  in  the  growing  cold.  They  are  now 
dead  worlds  revolving  about  a  dying  sun. 


/x^"- xN\ 


I    .        N        :.'...*f^WKf   w..-        .•       /         s 

\\:<::r  :>•'// 

:.       .?•%.  • ••  .•'  tr 


FIG.  90.  —  SOLAR  SYSTEM  AS  IT  is  NOW 

In  the  course  of  millions  of  years  the  sun  itself  cools  to  its 
centre,  and  shrinks  till  it  is  but  a  shadow  of  its  former  self.  All 
forms  of  energy  die  away,  the  times  of  rotation  of  the  planets 
and  moons  become  equal  to  their  periods  of  revolution,  and  the 
entire  system  is  dead  and  cold. 

Lord  Byron  once  wrote,  of  this  period  : 

"  I  had  a  dream  which  was  not  all  a  dream. 
The  bright  Sun  was  extinguished,  and  the  stars 
Did  wander  darkling  in  the  eternal  space, 
Rayless  and  pathless,  and  the  icy  Earth 
Swung  blind  and  blackening  in  the  moonless  air." 


COSMOGONIES,  AND  NEBULAR  HYPOTHESIS     189 

The  other  nebulae  which  surrounded  it  have  passed  through 
the  same  stages  and  are  also  dead.  The  Universe  is  one  vast 
cemetery  of  dead  suns  and  worlds. 

"  Time  was,  time  is,  and  time  shall  be  no  more. " 

Such  is  the  evolutionary  history  of  our  Universe  which  has 
been  theoretically  built  up  on  the  most  modern  form  of  Crea- 
tionism.  It  is  not  all  true,  yet  it  probably  contains  more  truth 
than  any  of  its  predecessors.  And  it  is  perhaps  about  as  near 
the  truth  as  the  majority  of  us  are  able  to  get  without  being 
overwhelmed  by  the  awful  realities  of  eternal  time  and  infinite 
space. 

Many  of  those  who  pursue  the  truth  wherever  it  may 
lead  them,  regardless  of  prejudices  and  consequences,  have 
changed  and  enlarged  the  original  Nebular  Theory  to  make 
it  fit  in  with  the  discoveries  of  the  nineteenth  century.  When 
thus  changed  it  is  no  longer  in  the  third  stage  of  Evolution- 
ary Creationism,  but  rests  entirely  on  the  fourth  and  last  basis 
of  Modern  Evolutionism.  It  will  be  dealt  with  in  Chapter 
XVIII. 


CHAPTER  XVII 

THEOKIES  AND    DISCOVERIES    MODIFYING   THE   NEBULAR 

HYPOTHESIS 

"  We  must  bear  in  mind  that  scientific  hypotheses  as  to  the  underlying  causes 
of  phenomena  are  subject  to  the  law  of  evolution,  and  have  their  birth,  maturity, 
and  decay.  Theory  necessarily  succeeds  theory,  aud  while  no  hypothesis  can  be 
looked  upon  as  expressing  the  whole  truth,  neither  is  any  likely  to  be  destitute 
of  all  truth  if  it  sufficiently  reconciles  a  large  number  of  observed  facts. 

"  The  notion  that  we  can  reach  an  absolutely  exact  and  ultimate  explanation 
of  any  group  of  physical  effects  is  a  fallacious  idea.  We  must  ever  be  content 
with  the  best  attainable  sufficient  hypothesis  that  can  at  any  time  be  framed  to 
include  the  whole  of  the  observations  under  our  notice.  Hence  the  question, 
'  What  is  electricity  ?  '  no  more  admits  of  a  complete  and  final  answer  to-day  than 
does  the  question,  '  What  is  life?  '  Though  this  idea  may  seem  discouraging,  it 
does  not  follow  that  the  trend  of  scientific  thought  is  not  in  the  right  direction. 
We  are  not  simply  wandering  round  and  round,  chasing  some  illusive  will-o'-the- 
wisp,  in  our  pursuit  after  a  comprehension  of  the  structure  of  the  Universe. 
Each  physical  hypothesis  serves  as  a  lamp  to  conduct  us  a  certain  stage  on  the 
journey.  It  illuminates  a  limited  portion  of  the  path,  throwing  a  light  before  and 
behind  for  some  distance,  but  it  has  to  be  discarded  and  exchanged  at  intervals, 
because  it  has  become  exhausted  and  its  work  is  done."  —  Professor  J.  A.  Fleming. 

SINCE  the  original  Nebular  Theory,  outlined  at  the  close  of 
the  preceding  chapter,  was  formulated  by  Kant  and  La- 
place, great  additions  have  been   made  to  our   knowledge   of 
natural   laws.     And  these   additions   have   led   to   important 
modifications  of  the  theory. 

Some  of  the  most  important  of  these  modifying  discoveries 
and  theories  will  now  be  briefly  sketched.  The  reader  is  par- 
ticularly desired  to  bear  in  mind  Professor  Fleming's  words  as 
given  at  the  head  of  this  chapter. 

MATTER  AND  ETHER 

Although  it  cannot  be  proved,  it  is  now  generally  agreed  that 
substance,  or  matter,  exists  in  two  forms,  one  of  which  has 
many  subdivisions  while  the  other  appears  to  be  homogeneous. 


MODIFYING  THE   NEBULAR   HYPOTHESIS    191 

I.  The  first  and  most  obvious  division  contains  all  forms  of 
what  may  be  termed  ponderable   matter,  or,  for  convenience, 
simply  matter.     It  includes  all  kinds  of  substance  which  are 
obvious  to  our  senses.     All  solids,  liquids,  and  gases  belong  to 
this  division. 

This  sensible  matter  is  supposed  to  consist  of  a  variety  of 
very  small  but  perfectly  distinct  atoms.  Those  substances 
which  are  built  up  entirely  of  one  kind  of  atom  are  known  as 
elements,  while  those  containing  two  or  more  kinds  of  atoms 
are  known  as  compounds.  It  possesses  such  characteristics 
as  go  by  the  name  of  gravity,  inertia,  molecular  heat,  and 
chemical  affinity. 

II.  The  other  form   of   substance  may  be  termed  ethereal 
matter,  though  it  is  commonly  known  as  ether  ^     It  does  not 
consist  of  a  variety  of  atoms,  like  the  ponderable  matter  just 
mentioned.     It  is  practically  imponderable   and  is  absolutely 
imperceptible  to  the  senses.     We  have  therefore  only  indirect 
proofs  of  its  existence. 

According  to  one  of  the  most  modern  theories  concerning  this 
ether  it  is  composed  of  particles  which  are  very  much  smaller 
than  atoms  and  are  all  exactly  alike  in  every  respect.  These 
are  supposed  to  be  so  crowded  together  that  they  act  like  one 
continuous  substance.  It  has  also  been  likened  to  an  incon- 
ceivably thin  elastic  transparent  jelly,  filling  all  space  not  occu- 
pied by  ponderable  matter.  It  even  fills  the  spaces  between 
the  atoms  of  the  latter.  Its  existence  appears  to  be  proved  by 
the  action  of  gravitation  across  apparently  empty  spaces,  and 
also  by  its  wave-like  movements,  which  are  recognisable  as 
radiant  energy,  in  the  forms  of  chemism,2  light,  heat,  electricity, 
and  magnetism. 

Although  we  do  not  know  that  the  above  theory  is  correct, 
we  are  compelled,  by  reasoning  on  observed  phenomena,  to 
believe  that,  in  one  or  other  of  its  two  forms,  this  indestructible 

1  Not  the  bottled  ether  of  the  chemist,  but  the  luminiferous  ether  of  the 
astronomer. 

2  Producing  chemical  action.     It  is  also  known  as  actinism. 


192     HOW  TO   KNOW   THE   STARRY   HEAVENS 

substance,  or  matter,  fills  all  the  infinity  of  space,  without  any 
void  whatsoever. 

ATOMIC  THEORY 

The  science  of  chemistry  deals  with  ponderable  matter  only. 
It  has  ascertained  by  experiment  that  all  the  varied  substances 
known  are  composed  of  about  80  "  elementary  "  forms  of  matter. 
These  exist  either  separately,  as  elements,  or  combined,  in  cer- 
tain fixed  proportions,  to  form  chemical  compounds.  And  it 
has  explained  the  observed  phenomena  of  chemical  combination 
by  what  is  known  as  the  Atomic  Theory,  formulated  by  Dalton 
in  1808.  This  theory  is  that  each  of  the  elements  is  composed 
of  separate  and  infinitesimal  atoms.  These  are  all  supposed  to 
be  exactly  the  same  in  size,  weight,  shape,  and  properties,  but 
to  be  entirely  different,  in  size,  weight,  shape,  and  properties, 
from  the  atoms  of  any  other  element.  These  atoms  may  com- 
bine to  form  molecules,  but  seem  to  be  incapable  of  further 
analysis. 

PERIODIC  SYSTEM   OF  ELEMENTS 

Although  one  element  cannot  be  changed  into  any  other  ele- 
ment, yet  the  different  elements  do  not  appear  to  be  absolutely 
independent  of  one  another.  When  they  are  arranged  according 
to  their  combining  weights,  they  fall  naturally  into  family 
groups,  reminding  one  of  the  octaves  of  music,  where  the  eighth 
notes  are  related  to  one  another.  It  is  probable,  therefore,  that 
the  elements  are  not  ultimate  unchangeable  forms  of  matter, 
but  that  their  atoms  consist  of  variously  arranged  groups  of  one 
primitive  form  of  matter.  This,  when  uncondensed,  may  possi- 
bly form  the  substance  of  the  luminiferous  ether. 

THE  LAW  OF   SUBSTANCE 

One  of  the  most  far-reaching  of  modern  discoveries  is  that  of 
the  Law  of  Substance,  commonly  known  as  the  Law  of  the 
Conservation  of  Matter  and  Energy. 

Indestructibility  of  Matter.  —  One  half  of  this  law  of  sub- 


FIG.  91.  —  DUMB-BELL  NEBULA 
Lick  photograph. 


MODIFYING   THE   NEBULAR   HYPOTHESIS     193 

stance  was  discovered  much  earlier  than  the  other  half,  and  is 
still  known  as  the  Law  of  the  Persistence  or  Indestructibility 
of  Matter.  It  was  worked  out  experimentally,  with  the  balance, 
in  the  laboratory  of  Lavoisier,  as  early  as  1789.  This  first  half 
of  the  law  affirms  that  no  substance  is  ever  created  or  destroyed ; 
that  the  Universe  always  contains  exactly  the  same  quantity  of 
matter  ;  and  that  chemical  processes  do  not  increase  or  decrease 
its  quantity,  but  merely  change  its  condition. 

The  modern  science  of  Chemistry  has  been  largely  built  on 
this  half  of  the  law  of  substance,  and  it  is  now  accepted  by  all 
thinking  men. 

Conservation  of  Energy.  —  The  other  half  of  the  law  of  sub- 
stance is  known  as  the  Law  of  the  Persistence  of  Force  or 
Conservation  of  Energy.  It  was  worked  out  experimentally,  in 
the  workshop  of  Eobert  Mayer,  in  1842.  This  second  half  of 
the  law  of  substance  affirms  that  no  force  is  ever  created  or 
destroyed ;  that  the  Universe  always  contains  exactly  the  same 
amount  of  energy ;  and  that  chemical  and  mechanical  changes 
do  not  increase  or  decrease  its  amount.  They  merely  change 
its  condition  from  potential  to  actual,  or  from  actual  to  potential, 
leaving  the  sum  total  of  the  two  forms  of  energy  eternally  the 
same. 

The  physical  sciences  have  been  largely  built  on  this  half  of 
the  law  of  substance,  and  it  is  now  adopted  (either  as  a  fact  or 
as  a  working  hypothesis)  by  all  thinking  men. 

Actual  energy  manifests  itself  in  several  different  ways,  and 
one  kind  of  it  can  readily  be  changed  into  another.  Sound, 
heat,  light,  chemical  action,  electricity,  magnetism,  etc.,  are  all 
manifestations  of  energy,  and  any  one  of  them  can  be  converted 
into  any  other  without  actual  loss  of  energy. 

Some  one  may  here  say,  "  That  sounds  like  perpetual  motion, 
which  we  know  to  be  an  absurdity." 

It  is  true  that  perpetual  motion  is  an  absurdity  so  far  as 
machinery  constructed  by  man  is  concerned.  That,  however, 
is  not  because  any  of  the  energy  is  destroyed,  but  because  it  is 
turned  into  a  form  which  is  not  available  to  us.  It  still  exists, 

13 


194     HOW  TO   KNOW  THE   STARRY    HEAVENS 

and  accurate  measurement  will  show  that  there  is  no  loss  of 
energy  whatever. 

All  forms  of  energy  are  available  to  Nature,  so  that  the  Uni- 
verse as  a  whole  is  not  only  a  "  perpetual-motion  machine,"  but 
is  the  only  one  that  can  possibly  exist. 

SPECTROSCOPIC  DISCOVERIES 

The  discovery  of  the  dark  lines  in  the  solar  spectrum,  by 
Wollaston  and  Frauiihofer,  and  their  explanation  by  Kirchhoff 
and  others,  after  fifty  years  of  study,  have  put  the  atomic  theory 
on  a  solid  basis,  and  given  an  immense  help  in  solving  the 
riddle  of  the  Universe.  KirchhofFs  discovery,  in  fact,  deserves 
to  rank  with  the  discovery  of  the  law  of  gravitation  by  Newton. 
While  the  one  enables  us  to  weigh  the  suns  and  worlds  in  a 
balance,  and  to  find  out  their  past,  present,  and  future  move- 
ments, the  other  tells  us  what  they  are  made  of  and  the 
condition  they  are  in.  It  also  enables  us  to  detect  celestial 
phenomena  and  movements  that  the  telescope  by  itself  fails 
to  reveal.  These  achievements  have,  however,  been  already 
described  in  former  chapters,  so  need  not  be  further  dwelt  on 
here. 

KINETIC  (OR  VIBRATORY)  THEORY  OF  SUBSTANCE 

It  is  now  concluded  that  all  the  different  forms  of  energy 
—  gravitation,  sound,  heat,  light,  chemical  action,  electricity, 
and  magnetism  —  are  only  different  manifestations  of  one 
primitive  force.  This  is  commonly  conceived  to  be  a  vibratory 
motion  of  the  atoms  of  matter  dancing  to  and  fro  in  empty 
space,  and  influencing  one  another  at  a  distance  without  any 
medium. 

When  this  theory  is  examined,  however,  some  parts  of  it 
prove  not  only  mysterious,  but  improbable,  if  not  impossible. 
We  can  find  no  satisfactory  answer  to  the  question,  How  can  a 
thing  act  where  it  is  not  present  ? 


MODIFYING   THE   NEBULAR   HYPOTHESIS     195 


PYKNOTIC  (OR  CONDENSATION)  THEORY  OF  SUBSTANCE 

To  overcome  this  objection  it  has  been  suggested  that  all 
space  is  filled  with  a  simple  primitive  continuous  substance, 
which  has  a  tendency  to  contract  or  condense  around  infinitesi- 
mal centres.  These  centres  of  condensation  are  the  atoms  of 
ponderable  matter,  and  they  are  supposed  to  float  in  the  uncon- 
densed  matter,  which  goes  by  the  name  of  ether.  The  condensa- 
tion of  the  atoms  causes  a  stretching  of  the  surrounding  ether. 
The  efforts  of  the  atoms  to  complete  their  condensation  are 
therefore  opposed  by  the  resistance  of  the  ether  to  the  further 
increase  of  its  strain.  The  result  is  the  accumulation  of  an 
immense  amount  of  potential  energy  around  the  atoms,  and  of 
actual  energy  in  the  ether.  These  two  forms  of  energy  are 
continually  varying,  but  the  sum  of  them  is  ever  the  same. 
They  manifest  themselves  as  light,  heat,  gravitation,  and  all  the 
other  modes  of  motion  with  which  we  are  acquainted,  and 
thereby  produce  all  the  varied  phenomena  of  Nature. 

These  two  theories  of  substance  are  now  being  tried  in  the 
crucible  of  experiment  and  observation.  Fresh  facts  are  being 
discovered  every  day,  and  a  satisfactory  and  comprehensive 
theory  will  probably  be  constructed  before  very  long. 

ELECTRO-MAGNETIC  THEORY  OF  LIGHT 

Until  1872,  "chemism,"  light,  heat,  electricity,  and  mag- 
netism, were  almost  universally  regarded  as  separate  and  dis- 
tinct entities.  We  now  look  upon  them  as  merely  sensations 
or  effects  due  to  one  solitary  form  of  radiant  energy,  which  is 
given  off  by  all  radiant  suns,  and  manifests  itself  differently 
according  to  the  way  in  which  we  observe  it. 

According  to  Maxwell's  electro-magnetic  theory  of  light 
(which  is  now  generally  accepted),  heat,  light,  magnetism,  etc., 
are  simply  different  manifestations  of  electricity  generated  and 
sent  out  by  the  huge  electrical  machines  which  we  know  as 
suns  or  stars.  They  are,  in  fact,  due  to  stresses  and  strains  in 
the  luminiferous  ether. 


196     HOW   TO   KNOW  THE   STARRY  HEAVENS 

It  will  be  seen  that  the  tendency  now  is  not  to  seek  for 
mechanical  explanations  of  electrical  phenomena,  but  to  look 
for  electrical  explanations  of  mechanical  phenomena. 

ULTRA-ATOMIC   (OR  ELECTRONIC)   THEORY 

Kathode  Rays.  —  If  a  wire  which  carries  a  current  of  elec- 
tricity be  cut  in  two,  and  the  ends  kept  apart,  the  flow  is  of 
course  stopped.  But  if  the  cut  ends  (or  terminals)  are  inside 
a  closed  glass  vacuum-tube,  the  current  leaps  across  the  almost 
empty  space,  and  the  tube  is  filled  with  a  phosphorescent  glow. 
Sir  W.  Crook es,  and  Professor  Thomson,  after  years  of  experiment 
with  these  vacuum-tubes,  concluded  that  this  cold  light  is  due 
to  a  torrent  of  small  negatively  electrified  particles  of  radiant 
matter,  chipped  off  the  "  kathode  "  or  negative  terminal. 

These  kathode  particles  resemble  ordinary  matter  in  possess- 
ing inertia,  and  will  turn  a  toy  windmill  when  they  strike  it. 
By  allowing  some  of  them  to  pass  through  a  slit  in  a  mica 
diaphragm,  and  then  to  skim  along  the  surface  of  a  screen 
coated  with  zinc  sulphate,  their  course  becomes  visible  (in  the 
dark)  to  the  naked  eye.  Ordinarily  the  particles  travel  in  a 
straight  line,  so  that  their  course  resembles  a  straight  jet  of 
steam.  But  when  a  magnet  (or  a  plus  pole)  is  brought  near, 
the  jet  of  luminous  particles  bends  toward  it,  as  a  horizontal 
jet  of  water  bends  toward  the  earth.  The  amount  of  the 
deflection  depends  on  the  strength  of  the  attracting  current, 
the  mass  of  the  particles,  and  the  speed  at  which  they  travel. 
It  has  thus  been  ascertained  that  the  particles  are  from  700  to 
1000  times  less  (in  mass)  than  an  atom  of  hydrogen,  and  that  < 
they  travel  at  a  speed  comparable  with  that  of  light.  The 
torrent  gives  a  negative  electric  charge  to  any  bodies  it  may 
strike,  and  it  appears  to  be  virtually  an  electric  current. 
Metals  are  transparent  to  it,  and  even  after  passing  through 
them  it  affects  a  photographic  plate  more  powerfully  than 
ordinary  light. 

Rontgen  or  X  Rays.  —  Any  object  struck  by  these  kathode 
particles  gives  off  what  are  thought  to  be  invisible  ether  waves. 


FIG.  92.  —  NOVA  PERSEI,    1901.     SHOWING  MOVEMENT    OP 
SURROUNDING  NEBULOSITY 
Lick  photographs. 

(a)  Nov.  7-8,  1901.     Exposure  7  h.  19  m. 

(6)  Jan.  31  and  Feb.  2,  1902.     Exposure  9  h.  45  m. 


MODIFYING  THE   NEBULAR   HYPOTHESIS     197 

These  are  commonly  known  as  Rontgen  or  X  Rays.  They 
radiate  from  their  source  in  straight  lines,  and  are  not  deflected 
by  a  magnet  or  electrical  field.  They  can  pass  through  wood, 
metal,  leather,  and  flesh  without  losing  their  power  of  affecting 
a  photographic  plate.  They  do  not  impart  negative  charges  to 
the  bodies  on  which  they  fall,  but  they  discharge  charged 
bodies  by  making  gases  better  conductors  of  electricity. 

Becquerel  Rays  (Alpha,  Beta,  and  Gramma). — Besides  the  two 
kinds  of  STIMULATED  radio-activity  just  described,  some  forms  of 
matter  possess  a  SPONTANEOUS  radio-activity.  All  compounds 
containing  the  heavy  elements  known  as  radium,  thorium, 
cerium,  and  (perhaps)  actinium,  give  out,  continuously  and 
spontaneously,  three  distinct  kinds  of  radiant  energy.  These 
are  known  as  — 

Alpha,     or  Atomic  Rays. 

Beta,       or  Kathodic  Rays,  and 

Gamma,  or  Rontgen  Rays. 

Alpha  or  Atomic  Rays.  —  These  are  the  most  noticeable  of 
the  spontaneous  radiations,  and  appear  to  consist  of  atomic 
projectiles  shot  out  in  all  directions  from  the  radio-active 
substances.  They  move  in  a  straight  line  with  a  velocity  of 
about  20,000  miles  per  second.  They  are  not  ordinarily  drawn 
aside  by  a  magnet  or  by  an  electrically  charged  body,  but  in 
a  very  stroDg  electrical  field  they  are  deflected  toward  the 
negative  pole.  The  direction  and  amount  of  this  deflection, 
with  a  current  whose  intensity  is  known,  prove  that  they  con- 
sist of  positively  electrified  atoms  whose  mass  is  two  or  three 
times  as  great  as  that  of  hydrogen  atoms.1  On  account  of  their 
great  size  they  cannot  pass  between  the  atoms  of  ordinary  mat- 
ter. They  can  therefore  be  stopped  by  a  sheet  of  paper.  They 
affect  a  photographic  plate  more  slowly  than  the  beta  particles, 
but  discharge  electrically  charged  bodies  more  quickly. 

1  They  may  possibly  be  atoms  of  the  newly  discovered  metal  helium,  whose 
atomic  weight  is  four  times  that  of  hydrogen.  This  element  is  never  found  except 
in  company  with  the  heavy  radio-active  elements  we  are  discussing,  and  its 
spectral  line  has  been  found  in  the  gaseous  emanations  from  radium. 


198     HOW  TO   KNOW  THE   STARRY  HEAVENS 

Beta  or  Kathodic  Rays.  — These  are  identical  with  the  kathode 
rays  of  the  Crookes  tube.  They  therefore  consist  of  negatively 
electrified  particles  about  2,000  times  smaller  (in  mass),  than 
the  alpha  atoms.  They  travel  in  a  straight  line  at  about  the 
speed  of  light.  They  can  pass  through  a  plate  of  platinum  or 
an  inch  of  solid  iron,  and  then  strongly  affect  a  photographic 
plate.  On  account  of  their  small  size  they  cannot  discharge 
electrified  bodies  so  quickly  as  the  more  ponderous  alpha 
projectiles. 

Gramma  or  Rontgen  Rays.  —  These  are  identical  with  the 
x  rays  of  the  Crookes  tube.  They  are  probably  ether-waves 
caused  by  the  breaking  up  of  the  atoms.  They  can  pass 
through  six  inches  of  iron  and  then  affect  a  'photographic 
plate. 

Of  all  the  radio-active  elements  yet  discovered,  radium  is  by 
far  the  most  active.  It  is  indeed  so  active  that  its  compounds 
are  measurably  warmer  than  surrounding  objects.  In  the  dark 
they  give  off  a  faint  light  like  that  of  a  glow-worm.  When 
they  are  placed  near  a  screen  covered  with  zinc  sulphide,  the 
impact  of  the  bombarding  projectiles  on  the  zinc  crystals  gives 
a  display  which  (seen  through  a  microscope)  resembles  a  sky 
full  of  shooting-stars. 

The  available  evidence  seems  to  show  that  those  elements 
whose  atoms  are  very  heavy  and  complex  were  built  up  under 
conditions  very  different  from  the  present  ones,  and  are  now 
•very  slowly  disintegrating,  by  stages,  into  lighter  and  simpler 
elements.1  This  would  explain  the  apparently  inexhaustible 
supply  of  energy  possessed  by  the  radio-active  elements. 

In  this  connection  Professor  R  A.  Millikan  says :  — 

"  The  disintegration  of  a  gram  of  uranium,  or  thorium,  or  radium, 
sets  free  at  least  a  million  times  as  much  energy  as  that  which  is 
represented  in  any  known  chemical  change  taking  place  within  a 
gram  weight  of  any  known  substance.  The  experiments  of  the  last 

1  This  would  be  analogous  to  the  complex  molecules  which  are  built  up  by 
living  organisms  (at  the  expense  of  solar  energy)  only  to  disintegrate  into  simpler 
ones  at  the  first  available  opportunity. 


MODIFYING  THE   NEBULAR   HYPOTHESIS     199 

eight  years  have  then  marked  a  remarkable  advance  in  science,  in 
that  they  have  proved  the  existence  of  an  immense  store  of  sub-atomic 
energy."  l 

Glowing  metals  and  other  hot  bodies  give  off  radiant  matter 
somewhat  similar  to  some  of  the  forms  described  above.  Cold 
metals  do  the  same  when  they  are  exposed  to  ultra-violet  light. 
It  is  possible  that  all  forms  of  matter  may  emit  similar  particles 
all  the  time. 

It  would  be  absurd  to  suppose  that  these  bodies  can  give  out 
either  radiations  or  particles  continuously  without  any  loss  of 
energy  or  weight,  but  the  loss  may  sometimes  be  so  small  that 
they  may  appear  to  do  so.  In  some  cases  the  radiations  may 
have  been  previously  received  from  the  Sun,  and  the  vibrations 
changed  to  a  rate  which  our  senses  or  instruments  are  capable 
of  perceiving  or  registering. 

The  particles  of  which  most  radiant  matter  is  composed  are 
variously  known  as  electrical  corpuscles,  electrons,  ions,  or 
negative  particles.  It  is  worthy  of  note  that  different  sub- 
stances do  not  give  out  different  kinds  of  corpuscles,  but  that 
the  latter  appear  to  be  all  alike,  whatever  their  source. 

An  atom  of  hydrogen  is  the  smallest  of  all  known  atoms.  In 
decomposing  water  by  electricity,  an  atom  of  hydrogen  carries 
a  certain  definite  and  indivisible  charge  of  electricity,  which  is 
known  as  an  electron.  Now  it  has  been  found  that  one  of  our 
newly  discovered  corpuscles  [although  it  is  from  700  to  1000 
times  less  (in  mass)  than  an  atom  of  hydrogen]  carries  the  same 
amount  of  electricity.  This,  however,  is  always  what  is  known 
as  a  negative  charge.  Positively  charged  corpuscles  have  not 
yet  been  discovered,  and  they  probably  do  not  exist  in  a  free 
state. 

It  is  possible  that  all  the  various  forms  of  "  elementary " 
atoms  composing  ponderable  matter  are  built  up  of  concentric 
layers  of  corpuscles,  the  layers  being  alternately  positive  and 

1  "Recent  Discoveries  in  Radiation,"  Popular  Science  Monthly,  April,  1904. 
Perhaps  this  advance  will  help  to  set  at  rest  the  long-standing  dispute  between 
the  astronomers  and  the  geologists  as  to  the  duration  of  geologic  times. 


200     HOW   TO   KNOW   THE   STARRY   HEAVENS 

negative.  In  this  case  we  may  assume  that  the  outside  layer 
is  always  composed  of  negative  corpuscles,  so  that  the  atoms 
all  attract  one  another  at  a  distance,  but  mutually  repel  when 
close  together,  especially  when  they  are  of  the  same  sign.  For 
otherwise  the  atoms  would  mingle  and  lose  their  individuality. 
If  this  is  so,  then  the  vibrations  which  produce  light,  etc.,  are 
not  vibrations  of  the  atom  itself,  but  of  the  electrons  or  cor- 
puscles of  which  it  is  composed. 

According  to  this  Electronic  Theory,  electrons  or  corpuscles 
are  the  ultimate  particles  of  which  all  kinds  of  atoms  consist. 
The  atoms  themselves  are  "  star-clusters  "  of  electrons  in  stable 
orbital  motion  at  planetary  distances  from  one  another.  On 
this  hypothesis  a  cluster  of  about  700  electrons  forms  an  atom 
of  hydrogen.  An  atom  of  oxygen  contains  about  11,200  of 
them.  An  atom  of  gold  contains  about  137,200.  And  so  on. 

It  appears  probable  that,  besides  the  corpuscles  which  are 
built  up  into  atoms,  there  are  such  vast  quantities  of  free  nega- 
tive corpuscles  that  they  fill  all  space .  to  saturation.  If  so, 
then  the  luminiferous  ether  itself  consists  of  these  electrons  or 
negative  particles.1 

1  This  Electronic  or  Ultra-Atomic  Theory  appears  to  have  needlessly  alarmed 
many  people,  who  think  that  it  will  lead  to  the  overthrow  of  the  Atomic  Theory 
and  of  the  Law  of  Substance.  They  may  perhaps  be  set  at  ease  by  the  following 
words  by  Sir  Oliver  Lodge,  F.R.S.,  at  the  close  of  an  article  on  "  Radium  and  its 
Lessons."  He  says: 

"  Let  me  conclude  by  asking  readers  to  give  no  ear  to  the  absurd  claim  of 
paradoxers  and  others  ignorant  of  the  principles  of  physics,  who,  with  misplaced 
ingenuity,  will  be  sure  to  urge  that  the  foundations  of  science  are  being  uprooted, 
and  long-cherished  laws  shaken.  Nothing  of  the  kind  is  happening.  The  new 
information  now  being  gained  in  so  many  laboratories  is  supplementary  and  stim- 
ulating, not  really  revolutionary,  nor  in  the  least  perturbing  to  mathematical 
physicists,  whatever  it  may  be  to  chemists ;  for  on  the  electric  theory  of  matter  it 
is  the  kind  of  thing  that  ought  to  occur.  And  one  outstanding  difficulty  about 
this  theory,  often  previously  felt  and  expressed  by  Professor  Larmor,  —  that 
matter  ought  to  be  radio-active  and  unstable  if  the  electric  theory  of  its  constitu- 
tion were  true,  —  this  theoretical  difficulty  is  being  removed  in  the  most  brilliant 
possible  way." — Nineteenth  Century  Magazine,  July,  1903, 


FIG.  93.  —  SPECTRA  OF  NOVA  PERSEI,    SHOWING  CHANGES 
Yerkes  Observatory. 


Feb.  27. 
Feb.  28. 


Mar.  6. 
Mar.  15. 


r.  28. 


MODIFYING  THE   NEBULAR   HYPOTHESIS     201 


REPULSION  OF  LIGHT 

It  has  been  discovered,  first  by  theory  and  afterward  by  ex- 
periment, that  when  the  vibrations  of  light  strike  any  object 
they  push  against  it  with  a  force  which  varies  according  to  the 
square  of  its  diameter.  And  as  the  attraction  of  gravitation, 
acting  on  the  same  object,  varies  as  the  cube  of  its  diameter,  it 
follows  that,  if  the  object  is  small  enough,  it  will  be  violently 
repelled  from  the  source  of  light. 

These  facts  form  a  simple  and  sufficient  explanation  of  many 
physical  phenomena,  both  terrestrial  and  celestial,  and  have 
been  several  times  alluded  to  in  the  preceding  chapters. 

EVOLUTION  OF  LIFE,   ATOMS,  AND  WORLDS 

About  the  beginning  of  the  nineteenth  century,  Goethe, 
Lamarck,  and  some  other  naturalists  rejected  the  theory  of  the 
special  creation  of  the  different  species  of  plants  and  animals. 
They  contended  that  all  have  developed,  by  natural  means, 
from  the  simplest  forms  of  life,  which  originally  came  from 
non-living  matter.  This  development,  they  claimed,  was  car- 
ried on  by  means  of  the  interaction  of  heredity  and  adaptation. 

In  1859  Charles  Darwin  proved  the  truth  of  this  theory  of 
descent,  as  far  as  such  a  theory  is  capable  of  proof,  and  showed 
that  it  was  caused  by  a  struggle  for  existence  and  the  resulting 
natural  selection  by  the  survival  of  the  fittest. 

The  late  Herbert  Spencer  first  recognised  that  the  same  law 
of  evolution  dominates  the  entire  Universe.  He  showed  that 
its  transformations  are  exhibited,  not  only  by  the  Universe  as 
a  great  whole,  but  in  all  its  details.  They  can  be  traced  in  the 
Solar  System,  and  in  the  inorganic  Earth ;  in  the  organic  world 
as  a  whole,  and  in  each  individual  organism ;  in  society  at 
large,  and  in  the  individual  mind.  They  are  also  clearly  recog- 
nisable in  all  the  products  of  social  activity. 

From  suns  and  worlds  to  molecules  and  atoms,  all  things 
struggle  for  existence,  and  survive  or  perish  according  to  their 


202    HOW  TO  KNOW  THE  STARRY  HEAVENS 

fitness.  The  reason  why  atoms  and  suns  are  in  a  state  of 
stable  equilibrium  that  seems  to  be  the  result  of  mental  con- 
trivance is  that  all  the  forms  which  do  not  possess  that  "  fitness 
to  survive "  are  promptly  changed  into  forms  that  are  not 
mutually  interfering.  In  both  atoms  and  suns  there  is  an 
unconscious  struggle  for  existence  leading  to  an  equally  un- 
conscious survival  of  the  fittest. 

PRIMEVAL  TIDES 

The  shape,  size,  condition,  and  movements  of  any  heavenly 
body  are  due  to  the  forces  which  act  (or  have  acted)  upon  the 
individual  particles  of  which  it  consists.  The  mathematical 
study  of  these  forces  has  led  to  the  discovery  that  the  develop- 
ment of  suns  and  worlds  is  largely  controlled  by  tidal  action 
while  they  are  still  in  a  gaseous  or  liquid-gaseous  condition. 

In  a  solar  system  like  ours,  and  in  a  binary  system  or  star- 
cluster,  the  various  bodies  are  moving  with  a  certain  regularity 
along  practically  non-interfering  lines.  .  This  is  not  the  result 
of  skilful  manoeuvring  by  clever  world-pilots.  There  is  no  one 
steering  them  out  of  danger.  Their  apparent  security  is  simply 
due  to  the  fact  that  interfering  movements  soon  lead  to  catas- 
trophes which  effectually  remove  the  insecure  members  of  the 
family  circle,  but  leave  the  others  alone.  It  is  simply  an  in- 
stance of  the  struggle  for  existence  and  of  the  survival  of  the 
fittest. 

But  the  sovereign  suns  which  roam  at  large  through  space 
all  appear  to  be  flying  at  random.  So  far  as  we  know  they 
have  no  regular  orbit,  but  each  one  is  apparently  dashing 
blindly  in  the  direction  of  least  resistance  to  the  surrounding 
centres  of  attraction.  They  may  be  likened  to  a  cloud  of 
mosquitoes  or  a  vast  swarm  of  bees. 

Let  us  suppose  that  two  gaseous  suns  are  approaching  one 
another  from  opposite  directions.  Each  one  is  sailing  majes- 
tically along  in  a  practically  straight  line,  and  at  the  same 
time  is  serenely  spinning  around  on  its  axis. 

If  there  were  pilots  on  board,  who  could  control  them  by 


MODIFYING  THE   NEBULAR  HYPOTHESIS    205 

means  of  a  steering-apparatus,  they  would  probably  get  in  a 
flurry  sometimes,  steer  wildly,  and  cause  a  collision.  But  as 
there  is  no  one  in  charge  to  make  trouble,  an  "accident"  of 
this  kind  seldom  takes  place. 

It  seems  natural  to  suppose  that  if  the  two  suns  do  not 
actually  collide  they  will  pass  each  other  and  go  on  their  way 
as  though  they  had  never  met.  This  is  the  case,  indeed,  when 
they  are  a  great  distance  apart.  But  if  they  pass  very  near  to 
one  another  the  law  of  gravitation  compels  them  to  salute  each 
other.  Their  original  speed  is  added  to  by  their  mutual  attrac- 
tion. They  swing  in  toward  one  another,  pass  like  a  flash,  and 
go  off  on  a  curve  which  soon  becomes  practically  a  straight 
line. 

So  far  the  only  effect  noticed  has  been  a  change  of  direction. 
But  there  appear  to  be  other  changes  produced.  When  the 
two  bodies  were  making  their  bow  to  each  other,  the  central 
particles  in  each  of  the  suns  were  attracted  less  than  the 
nearest  particles,  but  more  than  the  farthest.  The  intensity 
of  the  pull  varies  in  the  inverse  ratio  to  the  square  of  the  dis- 
tance (see  Chapter  XV).  The  result  is  the  same  as  though 
the  nearest  and  farthest  particles  in  each  body  were  strongly 
repelled  from  one  another.  The  two  suns  therefore  lose  their 
original  orange-shape  (due  to  attraction  modified  by  rotation) 
and  become  more  or  less  the  shape  of  a  pear.  After  they  have 
passed  each  other,  never  to  meet  again,  they  continue  to  rotate 
as  before,  and-  the  small  end  of  each  pear-shaped  star  swings 
around  as  a  mighty  tidal  wave.  The  centrifugal  tendency  is 
so  strong  that  the  two  ends  gradually  draw  the  central  matter 
to  them,  forming  a  dumb-bell  arrangement  which  finally  breaks 
in  two.  The  result  is  that  each  star  becomes  a  "  binary  "  or 
double  star,  in  which  both  of  the  partners  rotate  in  the  same 
direction  as  that  in  which  the  parent  moved.  The  telescope 
and  spectroscope  show  the  heavens  to  be  crowded  with  such 
binary  systems.  In  some  cases  the  rotation  is  so  rapid  as  to 
show  that  the  partners  are  still  clinging  together  like  Siamese 
Twins. 


204    HOW  TO  KNOW  THE   STARRY  HEAVENS 

The  binary  system  known  as  V  Puppis  appears  to  be  in  this 
stage  of  evolution.  The  combined  mass  of  the  two  partners  is 
equal  to  that  of  66,000  worlds  like  ours,  and  they  are  still  in 
a  distended  gaseous  state.  Yet  they  are  shown  to  revolve  (or 
rotate)  in  the  remarkably  short  period  of  35  hours. 

The  theory  of  tidal  evolution  (on  which  the  above  descrip- 
tion of  "puppation"  is  founded)  was  demonstrated  mathe- 
matically by  Professor  George  Darwin  from  an  examination 
of  the  interaction  between  the  tides  and  the  motions  of  the 
Earth  and  Moon. 


I 


•     » 


• 


CHAPTER    XVIII 

MODIFICATIONS  OF  THE  NEBULAR  THEORY 

"  Worlds  on  worlds  are  rolling  ever, 

From  creation  to  decay, 
Like  the  bubbles  on  a  river, 

Sparkling,  bursting,  borne  away."  —  P.  B.  Shelley. 
"  Without  beginning,  aim  or  end  ; 
Supreme,  incessant,  un begot ; 
The  systems  change,  but  goal  is  not, 
Where  the  Infinities  attend."—  G.  Sterling. 

IMMORTALITY  OF  THE  UNIVERSE 

THE  discoveries  and  speculations  I  have  just  sketched,  and 
others  which  have  not  been  mentioned,  have  led  to  some 
important  modifications  and  developments  of  the  original  Neb- 
ular Theory. 

Among  other  changes,  the  foundation  of  Creationism  has 
been  rejected  by  the  foremost  minds,  so  that  the  whole  theory 
is  evolutionary.  We  now  recognise  no  beginning  and  acknowl- 
edge no  end.  We  can  conceive  of  no  space  which  is  not  occu- 
pied by  matter  in  one  or  other  of  its  two  forms.  We  can  admit 
of  no  exhaustion  of  energy  leading  to  a  dead  Universe.  We 
have  come  to  the  conclusion  that  nothing  exists  apart  from 
matter  and  its  energies.  Mind,  in  the  form  of  desires  and  in- 
clinations, exists  not  only  throughout  the  animal  and  vegetable 
kingdoms,  but  likewise  in  so-called  dead  matter.  Even  the 
molecules,  atoms,  and  corpuscles  have  a  kind  of  sensation  and 
will. 

-FROM  DEATH  UNTO  LIFE" 

The  secret  of  the  perpetual  youth  of  the  Universe  appears  to 
lie  in  the  fact  that  the  millions  upon  millions  of  heavenly 


206     HOW  TO   KNOW  THE   STARRY   HEAVENS 

bodies  do  not  move  in  paths  of  eternal  regularity,  but  that  they 
interfere  with  one  another.  This  causes  their  orbits  to  be 
changeable,  and,  to  some  extent,  irregular.  The  result  is  that 
every  once  in  a  while  two  of  them  come  into  violent  collision, 
thus  changing  a  large  amount  of  potential  into  actual  energy. 

If  two  trains  travelling  at  the  speed  of  a  mile  a  minute  were 
to  meet  "  head  on,"  it  is  obvious  that  an  enormous  amount  of 
latent  energy  would  be  turned  to  actual  energy.  If  they  were 
travelling  750  times  faster  (which  is  the  speed  at  which  our 
Sun  is  going  toward  Vega),  the  amount  of  actual  energy  pro- 
duced by  the  shock  would  be  540,000  times  greater  (  =  750x 
750).  And  if  the  two  trains  were  to  be  loaded  down  till  they 
each  equalled  our  Sun  in  weight,  the  energy  produced  would 
be  proportionately  increased.  It  would  be  equal  to  that  caused 
by  a  direct  collision  between  our  Sun  and  another  star  of  the 
same  mass  and  travelling  at  the  same  speed; 

The  result  of  such  a  collision  could  be  calculated  mathe- 
matically, but  the  most  vivid  imagination  could  not  picture  it. 
Yet  there  are  suns  many  thousands  of  times  larger,  travelling 
at  a  speed  many  times  as  great,  and  apparently  liable  at  any 
time  to  come  together  with  a  crash,  or  to  glance  by  one  another 
with  a  result  almost  as  disastrous.  In  the  event  of  such  a 
collision  the  temperature  of  the  colliding  bodies  would  be  raised 
many  thousands  of  degrees.  They  would  expand  into  a  huge 
mass  of  thin  gas,  which  would,  in  time,  cool  off  to  the  tempera- 
ture of  space  (probably  about  —  230°  F.).  That  is  to  say,  the 
old  and  feeble  —  perhaps  dead  —  suns  would  spring  into  new 
youth  and  drift  away  as  a  more  or  less  diffuse  and  shapeless 
mass  of  glowing  gas.  This  would  gradually  assume  the  form 
of  a  rotating  spiral  nebula,  and  begin  once  more  the  great  drama 
of  evolution. 

This  picture  is  no  freak  of  the  imagination,  but  appears  to  be 
an  illustration  of  an  actual  fact.  In  all  probability  such  things 
have  always  been  taking  place,  are  taking  place  now,  and  will 
be  taking  place  when  our  Solar  System  has  for  ever  disappeared 
and  been  forgotten. 


MODIFICATIONS  OF  THE  NEBULAR  THEORY     207 

We  thus  see  that  the  evolution  of  suns  and  worlds  does  not 
necessarily  take  place  once  and  then  cease.  It  is  apparently 
repeated  over  and  over  again ;  here,  there,  and  everywhere.  As 
Ernst  Haeckel  says,  "  while  the  rotating  masses  move  toward 
their  destruction  and  dissolution  in  one  part  of  space,  others 
are  springing  into  new  life  and  development  in  other  quarters 
of  the  Universe." 

SOME  NEBULA  ARE  COLD 

The  Nebular  Theory  of  Laplace  was  that  the  nebulae  were 
extremely  hot,  and  rotated  in  lens-shaped  masses  which  threw 
off  regular  rings.  It  has  since  been  modified  for  both  theoreti- 
cal and  observational  reasons. 

In  the  first  place  it  is  evident  that,  however  hot  a  thin  nebula 
may  be  at  its  first  formation  (due  to  the  collision  of  mighty 
suns),  there  is  nothing  to  prevent  that  heat  from  radiating  away 
into  outer  space.  When  it  has  done  so  there  is  no  fresh  supply 
of  heat  to  draw  from  except  that  produced  by  the  action  of 
gravitation  drawing  its  spirals  back  into  condensed  centres,  and 
it  takes  millions  of  years  for  that  to  produce  any  great  increase 
of  temperature.  So  in  the  meantime  it  may  be  invisible  except 
at  the  outer  surface.  There  a  rain  of  negatively  electrified  cor- 
puscles (sent  out  by  the  myriads  of  radiant  suns)  probably 
causes  it  to  glow  with  a  cold  light  akin  to  many  other  corpus- 
cular radiations.1 

In  the  second  place  it  has  been  found  that  the  most  common 
form  of  nebula  seen  in  the  telescope  is  not  lens-shaped,  but 
spiral.  While  some  of  the  nebulae  (like  the  Great  Nebula  in 
Andromeda)  seem  to  be  condensing  into  a  vast  central  globe 
surrounded  by  tolerably  even  rings,  others  (like  the  Spiral 
Nebula  in  Canes  Venatici)  appear  to  be  condensing  around  a 
number  of  centres,  as  though  forming  a  social  system  or  star- 
cluster  (see  Figures  63,  64,  and  85). 

1  Among  these  may  be  mentioned  the  Solar  Corona,  the  tail  of  a  comet,  the 
Zodiacal  Light,  the  Gegenschein,  the  Aurora  Borealis  and  Australis,  St.  Elmo's 
fires,  the  phosphorescence  of  fishes  and  other  animals,  the  kathode  rays  of  our 
electricians,  etc. 


208     HOW  TO   KNOW  THE   STARRY  HEAVENS 


EVOLUTION  OF  SOLAR  SYSTEM 

When  our  Sun  was  in  its  first  nebular  stage  (perhaps  due  to 
collision),  it  was  probably  a  hot  and  more  or  less  spherical  mass 
of  inconceivably  thin  gas,  immensely  larger  than  the  whole  Solar 
System  is  now.  Its  particles  would  then  be  so  far  apart  that 
the  most  perfect  vacuum  we  can  produce  would  be  dense  in 
comparison.  Seen  from  a  great  distance,  it  would  probably  re- 
semble the  symmetrical  planetary  nebulae  which  are  revealed 
by  our  telescopes  (see  Figure  99). 

Presuming  that  this  planetary  nebula  was  not  interfered  with 
from  outside,  it  would  not  lose  its  regularity  of  form,  but  would 
gradually  condense  and  acquire  a  spiral  structure.  But  if  any 
wandering  stars  had  happened  to  pass  through  it,  it  might  for  a 
time  have  assumed  the  torn  explosive  appearance  of  the  great 
Trifid  Nebula  (see  Figure  67),  and  if  it  had  collided  with  a 
neighbouring  nebula  it  might  possibly  "have  acquired  an  irregu- 
lar form  like  that  of  the  Great  Nebula  in  Orion  (see  Figure  68), 
but  of  course  it  would  have  been  on  a  much  smaller  scale. 
Unless  very  much  scattered,  however,  the  mutual  attraction  of 
its  particles  would,  in  the  course  of  time,  again  bring  about  a 
regularity  of  form  and  structure. 

When  a  cricket-ball  is  struck  by  a  bat,  the  size,  shape,  and 
density  of  the  two  bodies,  and  their  position,  speed,  and  direc- 
tion, determine  the  flight  of  the  ball  and  the  subsequent  adven- 
tures it  may  meet  with.  In  the  same  way  the  colliding  bodies 
which  we  assume  to  have  given  birth  to  the  nebulous  mass 
under  consideration  must  have  received  a  certain  moment  of 
momentum  which  has  produced  every  peculiarity  now  possessed 
by  the  Solar  System  which  has  developed  from  it. 

After  the  explosive  energy  of  the  original  collision  had  pro- 
duced the  hypothetical  nebula,  every  gaseous  particle  in  it  was 
left  at  a  certain  temperature  and  in  a  certain  position.  It  was 
also  moving  in  a  certain  direction  with  a  certain  velocity.  The 
varied  and  irregular  nature  of  these  movements  led  to  innumer- 
able collisions  between  the  particles.  Some  of  the  energy  pos- 


MODIFICATIONS  OF  THE  NEBULAR  THEORY     209 

sessed  by  the  particles  was  thus  turned  to  heat,  which,  like  the 
original  heat  of  the  nebula,  radiated  away  into  outer  space.  The 
varied  movements  of  the  particles,  and  the  resulting  collisions, 
naturally  ended  in  the  survival  of  the  movements  which  did  not 
interfere  with  one  another.  The  mass  therefore  gradually  at- 
tained a  rotating  disc-like  form,  lying  on  a  plane  —  and  moving 
in  a  direction  —  determined  by  its  original "  moment  of  momen- 
tum." In  this  way  every  particle  moved  in  the  path  of  least 
resistance,  and  therefore  with  the  least,  expenditure  of  energy. 

By  this  time  the  temperature  of  the  nebula  was  very  much 
reduced  by  radiation  into  outer  space.  It  was  probably  invisi- 
ble except  near  the  surface,  where  the  radiant  energy  from  sur- 
rounding suns  made  the  light  surface-gases  glow  with  a  cold 
radiance  akin  to  that  of  the  kathode  rays  of  our  vacuum  tubes. 

The  dissipation  of  the  original  heat  by  radiation  led  to  the 
contraction  and  condensation  of  the  entire  mass.  The  decrease 
in  size  of  course  led  to  an  increase  of  the  density  and  speed  of 
rotation.  As  the  speed  was  greatest  toward  the  centre,  a  rotat- 
ing and  indrawing  spiral  was  the  result  (see  Figures  63  and  85). 

The  centre  of  this  spiral  became  a  vast  condensation  which, 
in  the  case  of  our  System,  drew  to  it  the  great  mass  of  the  neb- 
ula. Four  important  subordinate  centres  of  condensation  were 
also  formed,  as  well  as  a  great  number  of  lesser  ones.  These 
centres  gradually  absorbed  the  nebuldus  matter  around  them, 
and  thereby  increased  continuously  in  density. 

At  first  these  centres  of  condensation  did  not  have  any  rota- 
tion except  that  due  to  the  spiral  indrawing  revolution  of  the 
whole  mass.  That  is  to  say,  each  subordinate  centre  rotated  at 
the  same  speed  that  it  revolved.  But  as  its  particles  were  drawn 
in  toward  the  centre,  the  rotation  grew  more  rapid,  because  they 
had  a  shorter  journey  to  go  and  were  nearer  the  local  centre  of 
attraction.  All  the  subordinate  condensations,  therefore,  ac- 
quired an  ever-quickening  speed  of  rotation  in  the  same  direction 
as  the  revolution  of  the  whole  system.  In  the  case  of  the 
central  condensation  there  would  be  no  distinction  between 
rotation  and  revolution. 


210     HOW  TO   KNOW   THE   STARRY   HEAVENS 

As  the  particles  in  one  of  these  condensations  gradually  fell  in 
toward  its  centre,  their  friction  with  one  another  turned  a  large 
part  of  its  energy  into  heat.  So  much  heat  was  produced  in  this 
way  that  it  could  not  all  radiate  into  space.  Its  temperature, 
therefore,  rose,  slowly  but  steadily,  till  it  glowed  like  a  little  sun. 

As  a  result  of  the  changes  outlined  in  the  preceding  three 
paragraphs,  the  centres  of  condensation,  though  they  decreased 
in  size,  continually  increased  in  density,  speed  (of  rotation  and 
revolution),  and  heat. 

So  far  we  have  theoretically  traced  the  evolution  of  the  origi- 
nal nebula  into  a  system  of  thin,  hot,  and  bright  gaseous  planets 
revolving  around  a  huge  gaseous  centre.  This  central  conden- 
sation was  even  hotter  than  the  planets,  and  was  destined  in 
time  to  become  a  central  sun,  luminous  with  an  inconceivable 
intensity  of  heat. 

The  small  amount  of  nebulous  matter  which  still  remained, 
outside  the  planetary  centres,  was  gradually  drawn  in  by  them 
in  a  spiral  manner,  and  condensed  into  similar  but  smaller 
centres.  They  acquired  the  same  peculiarities  of  revolution 
and  rotation,  and  in  fact  repeated  the  original  history  of  the 
whole  system  on  a  smaller  scale.  This  appears  to  be  the  origin 
of  the  satellites  which  now  attend  the  larger  planets,  and  of  the 
rings  of  Saturn,  which  consist  of  innumerable  small  satellites. 

After  a  certain  amount  of  condensation  the  production  of 
frictional  heat  in  each  subordinate  centre  became  less  in 
amount.  As  the  radiation  continued  unchecked,  a  maximum  of 
heat  was  reached,  and  the  satellites  and  planets  then  began 
to  cool  off.  The  smaller  a  body  is,  the  larger  becomes  its 
surface  in  proportion  to  its  mass.  The  satellites  and  smallest 
planets  therefore  cooled  off  the  quickest.  Their  surfaces  solidi- 
fied and  became  solid  rock.  This,  being  a  bad  conductor  of  heat, 
somewhat  checked  their  loss  of  heat  by  radiation,  but  did  not  al- 
together prevent  it.  In  time  they  therefore  became  solid  to 
the  centre  and  gradually  approximated  to  the  temperature  of 
outer  space. 

The  larger  planets  are  now  going  through  the  same  cooling 


MODIFICATIONS  OF  THE  NEBULAR  THEORY 

and  solidifying  process.  But  the  central  Sun  is  still  producing 
about  as  much  heat  by  its  contraction  as  it  loses  by  radiation 
into  outer  space.  It  is  still  kept  in  an  incandescent  gaseous 
form  by  this  heat,  and  the  only  elements  in  it  which  have 
reached  a  solid  state  are  the  carbon  and  silicon  which  are  ex- 
posed to  the  cold  of  outer  space  above  a  certain  elevation  in 
its  atmosphere  of  metallic  gases.  Their  chilled  particles  are 
apparently  frozen  into  solid  incandescent  beads  that  form  the 
glittering  clouds  which  we  term  the  solar  photosphere. 

We  thus  see  that  the  heat  of  the  Sun  is  not  due  to  com- 
bustion. In  fact  combustion  is  like  organic  life,  —  it  cannot 
exist  above  or  below  a  certain  range  of  temperature.  The  heat 
of  the  Sun  is  at  present  far  too  great  to  allow  a  comparatively 
cold  process  like  combustion  to  take  place  anywhere  near  it. 

When  the  Sun  formed  the  bulk  of  the  original  nebula  its 
energy  was  all  potential.  Its  circling  particles  have  been 
falling  ever  since,  as  they  "  spirated "  around  the  centre  of 
gravity  of  the  mass.  Their  friction  in  falling  is  the  cause  of 
all  the  heat,  etc.,  which  has  since  been  produced  and  radiated. 
The  same  amount  of  heat  is  produced  by  the  friction  of  adja- 
cent particles,  whether  a  body  falls  rapidly  or  slowly,  directly 
or  indirectly.  As  the  Sun's  mass  has  not  changed  and  is 
known,  the  amount  of  potential  energy  changed  into  actual 
energy  can  be  computed.  The  radiant  energy  developed  by 
the  contraction  of  the  Solar  System  from  a  thin  nebula  is  equal 
to  the  energy  necessary  to  force  its  particles  asunder  and  return 
it  to  a  nebular  condition. 

After  the  contraction  had  gone  on  for  many  millions  of  years, 
the  various  planets  were  left  outside  of  the  central  condensing 
nucleus  in  the  way  already  described.  When  this  nucleus 
had  contracted  to  the  size  of  the  Earth's  present  orbit,  it  was 
still  about  12,000  times  thinner  than  our  atmosphere  at  the 
sea  level.  It  was  therefore  about  as  dense  as  the  most  perfect 
vacuum  we  can  make  with  an  air-pump.  As  it  continued  to 
shrink  in  size,  the  crowding  atoms  gradually  gave  rise  to  long 
heat-waves.  In  the  course  of  ages  the  short  light-waves  were 


HOW   TO   KNOW  THE   STARRY   HEAVENS 

also  produced,  turning  the  gaseous  mass  into  a  nebulous  sun. 
It  has  been  estimated  that  when  it  had  contracted  to  the  size 
of  the  present  orbit  of  Mercury  it  gave  off  about  one  eightieth 
of  its  present  radiant  energy,  and  when  the  continuous  con- 
traction had  reduced  it  to  a  thousand  times  its  present  size  its 
average  density  became  equal  to  that  of  our  atmosphere  at  the 
sea  level. 

As  the  light  gases  around  it  were  gradually  absorbed,  it 
slowly  changed  from  a  nebulous  star  into  a  brilliant  bluish- 
white  star,  and  afterward  into  a  yellowish-white  one.  It  is 
now,  on  the  average,  about  one  and  a  half  times  as  dense  as 
water.  Its  particles,  therefore,  fall  more  slowly,  but  produce 
more  friction  and  radiance. 

The  Sun's  contraction  in  size  has  now  fallen  off  to  about 
9  inches  per  day  —  say  a  mile  in  twenty  years.  This  is  so 
small  that  it  would  take  nearly  10,000  years  to  recognise  the 
shrinkage,  from  the  Earth,  with  our  present  instruments.  The 
maximum  of  heat  appears  to  have  been  passed,  and  the  light 
has  begun  to  wane  and  turn  yellow.  Some  day,  in  the  dim  and 
distant  future,  spasms  of  chemical  combination  will  take  place 
in  the  reddening  Sun.  It  will  gradually  liquefy  from  the 
centre  up  to  the  surface,  and  afterwards  solidify  from  the  sur- 
face down  to  the  centre.  It  will  then  cease  to  contract  and  to 
give  out  light  and  heat,  its  potential  energy  having  all  turned 
into  actual  energy,  and  radiated  away  into  space.  By  that  time 
its  average  density  will  have  probably  increased  to  about  twenty 
times  that  of  water. 

Although  satellites  or  moons  are  common  in  the  Solar  System, 
our  Moon  appears  to  be,  in  some  respects,  an  exceptional  one. 
It  is  very  much  larger,  in  proportion  to  the  Earth,  than  the 
satellites  of  the  other  planets  "are  when  compared  with  their 
primaries,  and  it  appears  to  have  had  a  different  origin.  Before 
it  was  born,  the  still  molten  Earth  appears  to  have  shrunk 
until  it  rotated  in  about  four  or  five  hours.  Its  equatorial  parts 
then  bulged  out  and  became  almost  weightless,  through  the 
centrifugal  force  resulting  from  the  rapid  rotation.  The  pull  of 


MODIFICATIONS  OF  THE  NEBULAR  THEORY     213 

the  Sun  on  this  weightless  mass  raised  a  huge  tide  on  the  Sun- 
ward side  of  the  Earth.  This  gradually  grew  in  size,  so  that 
the  Earth  assumed  something  of  a  pear  shape.  Finally  the 
rotation  became  so  rapid  that  the  tidal  wave  was  wrenched 
loose  from  the  main  body  of  the  Earth  and  became  our  Moon. 
This  tidal  parentage  is  known  as  a  "  fission  "  process,  and  was 
first  brought  to  light  by  Professor  George  Darwin. 

The  Earth  and  Moon,  while  still  very  close  together,  and 
still  molten,  produced  huge  tides  in  each  other.  The  Earth- 
raised  Moon-tides  acted  as  a  brake  on  the  Moon  and  checked 
its  rotation.  At  last  its  rotation  and  revolution  periods  became 
equal,  so  that  it  always  turned  the  same  face  to  the  Earth. 
The  Earth-tides  which  the  Sun  and  Moon  raised  acted  as  a 
brake  on  the  Earth  itself,  and  at  the  same  time  quickened 
the  forward  motion  of  the  Moon,  thereby  increasing  its  distance 
and  period. 

The  Earth  and  Moon  have  now  solidified,  so  that  the  tides 
on  the  Moon  have  ceased,  and  those  on  the  Earth  are  almost 


FIG.  95.  —  EARTH-TIDES,  IF  THE  DAY  AND  MONTH  WERE  EQUAL 
The  moon's  orbit  would  remain  constant. 

entirely  confined  to  the  large  open  bodies  of  water  on  its  sur- 
face. Yet  the  ocean  tides  still  continue  to  act  as  a  brake  to 
check  the  Earth's  rotation.  The  effect,  though  small,  is  cumu- 
lative, and  will  in  time  lengthen  the  Earth's  day  and  the 
Moon's  "  moonth  "  till  they  are  both  equal  to  about  55  of  our 
days.  The  lengthening  will  then  cease,  and  both  bodies, 
though  still  revolving  around  their  common  centre  of  gravity, 


214     HOW  TO   KNOW   THE   STARRY   HEAVENS 


will  be  relatively  immovable,  like  the  two  ends  of  a  dumb-bell, 
which  always  present  the  same  face  to  each  other. 

It  is  not  difficult  to  understand  that  the  tides  on  each  body 
should  act  as  a  brake  and  check  rotation.  But  it  is  not  so 
obvious  that  the  Earth- tides  should  pull  the  Moon  forward, 
and,  by  so  doing,  increase  the  size  of  its  orbit  and  the  period  of 
its  revolution.  The  principle  is  as  follows  : 

A  particle  of  matter  at  the  centre  of  the  Earth  is  pulled 
toward  the  Moon  with  a  certain  force.  A  particle  on  the  side 
nearest  the  Moon  is  pulled  with  a  greater  force.  And  a  parti- 


CUtt-TAttion 


)MOON 


FIG.  96.  —  ACCELERATION  OF  MOON  BY  FORWARD  PULL  OF  EARTH-TIDE 
Moon's  orbit  forms  a  slowly  opening  spiral. 

cle  on  the  side  away  from  the  Moon  is  pulled  with  less  force, 
and  is  therefore  (relatively)  repelled.  Consequently,  if  the 
Earth  rotated  at  the  same  speed  at  which  the  Moon  revolved, 
the  water  would  bulge  out,  not  only  on  the  side  nearest  the 
Moon,  but  also  on  the  side  farthest  from  it  (see  Figure  95). 

Owing,  however,  to  the  relative  rapidity  of  the  Earth's  rota- 
tion, the  tides  are  dragged  forward  by  molecular  friction.  The 
tide  nearest  the  Moon  is  therefore  a  little  in  front  of  the  line 
joining  the  centres  of  the  Earth  and  Moon  (see  Figure  96). 
It  is  the  forward  pull  of  this  Earth-tide  on  the  Moon  which 
quickens  the  motion  of  the  latter.  By  so  doing  it  very  slowly 
enlarges  the  Moon's  orbit  and  lengthens  its  period  of  revolution 
around  the  centre  of  gravity  of  the  Earth  and  Moon.  The 
action  (like  that  of  the  Earth-brake)  is  absolutely  imperceptible, 


MODIFICATIONS  OF  THE  NEBULAR  THEORY     215 

but  is,  at  the  same  time,  real  and  cumulative.  It  is  therefore 
only  a  question  of  time  for  great  changes  to  be  produced.  And 
of  time  there  is  no  end.1 

By  the  time  the  Earth  and  Moon  have  reached  the  dumb- 
bell stage,  the  lunar  tidal-friction  on  the  Earth  will  come  to  an 
end,  and  the  Moon's  path  will  cease  to  be  an  opening  spiraL 


FIG.  97.  — LOOP  IN  APPARENT  PATH  OF  MARS 

As  he  approaches  opposition,  his  direct  (easterly)  motion  amongst 
the  stars  slackens  and  soon  ceases.  He  then  appears  to  move  back 
towards  the  west.  After  opposition  this  retrograde  motion  also  slack- 
ens and  ceases.  His  direct  motion  then  recommences,  and  is  kept  up 
till  the  next  opposition.  He  thus  appears  to  make  a  loop  every  584  days. 

But  until  our  oceans  are  frozen  solid,  the  Earth's  solar  tides 
will  still  continue  to  act  as  a  brake  and  check  its  rotation.  So 
at  last  the  time  may  come  when  the  Earth's  day  will  equal  its 
year,  and  the  same  side  will  always  be  turned  toward  the  Sun.2 
It  is  rather  uncertain  what  changes  will  take  place  after  this 
stage  has  been  reached.  If  no  accident  should  happen,  in  the 
form  of  a  collision  with  some  outside  body,  the  probability  is 
that  the  satellites  and  planets  will  very  slowly  draw  in  toward 
their  centres  of  revolution.3  In  this  case  the  Moon's  path  will 

1  The  tide  which  is  away  from  the  Moon  tends  to  counteract  this  pull,  but  is 
too  weak  to  overcome  it  altogether. 

2  Mercury  and  Venus  appear  to  have  already  reached  this  stage. 

8  This  theory  is  based  upon  the  supposition  of  a  resisting  medium,  ethereal  or 
otherwise. 


216     HOW  TO   KNOW  THE   STARRY   HEAVENS 

gradually  get  smaller,  and  it  will,  in  time,  reach  the  surface  of 
the  Earth.  The  two  bodies  will  grind  each  other  into  super- 
heated gas,  which  will  afterward  cool  down  and  solidify.  The 
resulting  body  will  gradually  get  nearer  to  the  Sun,  and  even- 
tually grind  its  way  into  the  body  of  its  parent.1 

By  the  time  all  the  planets  have  returned  to  the  Sun,  and 
the  last  disturbance  has  subsided,  one  second  of  Eternity  will 


w 
FIG.  98.  —  DIAGRAM  SHOWING  CAUSE  OF  LOOP  IN  APPARENT  PATH  OF  MARS 

The  positions  of  Earth  and  Mars  given  at  intervals  of  ten  days.  Mars  appears  to  ad- 
vance amongst  the  stars  from  1  to  7,  to  retrograde  from  7  to  11,  and  then  to  advance  as 
before. 

have  passed  away  since  it  was  in  the  same  stage  before.  The 
Solar  System  will  then  be  represented  by  a  cold,  dark,  solid 
mass  containing  the  same  amount  of  matter  as  the  original 
nebula.  This  dead  hulk  will  drift  around  in  the  long  cold  star- 
light night  (like  a  derelict  on  the  broad  ocean)  until  some  simi- 
lar body  collides  with  it  and  turns  it  all  into  gas  again.  It  will 

1  If  this  theory  is  correct,  the  matter  of  which  our  bodies  are  composed  once 
formed  part  of  what  is  now  the  Sun,  and  will  in  time  return  to  it. 


MODIFICATIONS  OF  THE  NEBULAR  THEORY    217 

then  be  ready  to  live  its  life  over  again  and  give  birth  to  an- 
other Solar  System  perhaps  twice  as  large  as  the  present  one. 
These  cycles  will  presumably  take  place  an  infinite  number  of 
times,  the  same  matter  being  used  over  and  over  again  to  all 
eternity.  This  is  probably  true  not  only  of  the  future  but  of 
the  past ;  not  only  of  our  System  but  of  all  the  systems  of  the 
Universe;  not  only  of  our  Universe  but  of  all  the  universes 
that  may  exist  in  infinite  space. 

Now  that  I  am  through  with  the  Nebular  Hypothesis,  it  may 
be  well  to  remind  the  reader  again  that  it  is  still  an  unproved 
theory.  As  a  working  hypothesis  it  is  one  of  the  grandest  and 
most  valuable  theories  ever  reasoned  out  by  the  mind  of  man, 
and  in  its  main  features  it  is  almost  certainly  true  as  far  as  it 
goes.  But  from  the  very  nature  of  the  case  we  are  not  in  a 
position  to  prove  its  truth,  and  it  is  doubtful  if  we  ever  shall 
have  anything  except  indirect  evidence  concerning  it. 

SUMMARY 

I  cannot  do  better  than  close  this  chapter  with  a  summary, 
by  Ernst  Haeckel,  of  the  most  important  conclusions  at  which 
science  is  arriving  with  regard  to  the  constitution  and  evolu- 
tion of  the  Great  Cosmos.     It  is  as  follows: 

"  I.  The  extent  of  the  Universe  is  infinite  and  unbounded ;  it  is 
empty  in  no  part,  but  everywhere  filled  with  substance. 

"  II.  The  duration  of  the  Universe  is  equally  infinite  and  unbounded; 
it  has  no  beginning  and  no  end  :  it  is  eternity. 

II  III.    Substance  is  everywhere  and  always  in  uninterrupted  move- 
ment and  transformation  :  nowhere  is  there  perfect  repose  and  rigid- 
ity ;  yet  the  infinite  quantity  of  matter   and  of  eternally  changing 
force  remains  constant. 

"  IV.  This  universal  movement  of  substance  in  space  takes  the 
form  of  an  eternal  cycle  or  of  a  periodical  process  of  evolution. 

"  V.  The  phases  of  this  evolution  consist  in  a  periodic  change  of 
consistency,  of  which  the  first  outcome  is  the  primary  division  into 
mass  and  ether, — the  ergonomy  of  ponderable  and  imponderable 
matter. 


218     HOW  TO   KNOW  THE   STARRY  HEAVENS 

"  VI.  This  division  is  effected  by  a  progressive  condensation  of 
matter  as  the  formation  of  countless  infinitesimal  '  centres  of  conden- 
sation,' in  which  the  inherent  primitive  properties  of  substance  —  feel- 
ing and  inclination  —  are  the  active  causes. 

"VII.  While  minute  and  then  larger  bodies  are  being  formed  by 
this  pyknotic  process  in  one  part  of  space,  and  the  intermediate  ether 
increases  its  strain,  the  opposite  process  —  the  destruction  of  cosmic 
bodies  by  collision  —  is  taking  place  in  another  quarter. 

"  VIII.  The  immense  quantity  of  heat  which  is  generated  in  this 
mechanical  process  of  the  collision  of  swiftly  moving  bodies  represents 
the  new  kinetic  energy  which  effects  the  movement  of  the  resulting 
nebulae  and  the  construction  of  new  rotating  bodies.  The  eternal 
drama  begins  afresh." * 

1  "  The  Kiddle  of  the  Universe,"  pp.  242,  243. 


CHAPTER  XIX 

THE  MESSENGERS  OF  HEAVEN 

"  In  the  old  mythologies  the  Universe  consisted  merely  of  a  flat  World,  resting 
on  an  infernal  bowl,  and  covered  by  a  celestial  vault  or  canopy. 

"  The  Gods  who  ruled  this  mundane  Universe  generally  resided  on  the  top  of 
the  firmamental  dish-cover.  There  the  supreme  Deity  had  his  throne,  and  pre- 
sided over  the  councils  of  the  Gods. 

"  As  none  of  these  Gods  were  omniscient  or  omnipresent,  they  had  to  provide 
themselves  with  angelic  messengers  to  keep  them  posted  as  to  what  was  going  on 
iii  the  World  beneath,  and  to  carry  out  their  commands  when  they  had  come  to 
an  agreement  as  to  what  kind  of  left-handed  justice  should  be  meted  out  to  the 
mortals  below. 

"  These  messengers  of  heaven  were  no  loafers  round  the  throne  when  they  had 
a  duty  to  perform.  They  could  fly  on  the  wings  of  the  wind  and  outstrip  the 
fiercest  tempest. 

"  We  now  know  that  these  angelic  messengers,  like  the  Gods  they  were  supposed 
to  serve,  have  no  existence  except  in  the  imaginations  of  ignorant  and  superstitious 
people.  But  Science  has  revealed  to  us  that  there  are  messengers  of  heaven  more 
swift  than  the  wing-footed  Mercury,  more  restless  than  the  cloven-hoofed  Satan, 
and  more  long-lived  than  the  Father  of  the  Gods  himself." — A.  Zazel. 

VISITORS  FROM  AFAR 

IN  a  former  chapter  I  spoke  about  the  intense  loneliness  of 
our  Solar  System  in  the  midst  of  an  immense  multitude  of 
similar  systems  scattered  through  space. 

Is  there,  then,  no  physical  communication  between  the  vari- 
ous systems  of  worlds  ?  Are  they  entirely  and  for  ever  cut  off 
from  one  another?  Are  there  no  messengers  of  heaven  that 
can  serve  the  bidding  and  carry  the  messages  of  the  sunny-faced 
Gods  of  ethereal  space  ? 

Yes,  there  are  wanderers  who  go  from  star  to  star  and  from 
constellation  to  constellation.  There  are  beings  which  spend 
almost  an  eternity  in  going  to  and  fro  in  the  Universe,  and  in 
flying  up  and  down  through  it. 


220    HOW  TO   KNOW  THE   STARRY  HEAVENS 

A  VOYAGE  OF  FIVE  MILLION  YEARS 

In  order  to  learn  something  of  these  interstellar  wanderers, 
let  us  suppose  that  we  are  back  in  our  Chariot  of  Imagination, 
half  way  between  our  Sun  and  the  nearest  outside  star.  Let  us 
also  suppose  that  the  clock  has  been  turned  back  several 
millions  of  years ;  that  we  have  plenty  of  time  at  our  disposal ; 
and  that  we  have  laid  in  a  goodly  supply  of  Job's  patience. 

We  are  surrounded,  as  before,  by  countless  systems  of  worlds, 
but  they  are  all  in  the  remote  distance.  Let  us  move  around 
a  little,  and  see  if  our  immediate  neighbourhood  is  really  as 
empty  as  it  seems. 

With  the  naked  eye  we  might  have  to  look  for  a  long  time 
before  finding  anything  near  us.  But  with  suitable  instruments 
we  soon  find  that  although  there  are  no  suns  or  large  worlds  in 
our  neighbourhood  there  are,  here  and  there,  small  fragments  of 
hard  rock  floating  in  the  otherwise  empty  darkness.  These 
stones  sometimes  contain  a  large  percentage  of  iron,  and  greatly 
resemble  the  meteoric  stones  that  occasionally  fall  onto  our 
Earth.  They  vary  greatly  in  size,  some  of  them  being  no  larger 
than  grains  of  sand.  As  there  is  no  large  world  near  to  attract 
them,  they  are  destitute  of  weight  and  simply  float  in  empty 
space. 

If  we  search  long  enough  we  may  possibly  come  across  one  or 
two  similar  rock  masses  of  considerable  size,  for  there  does  not 
appear  to  be  any  break  between  these  "  pocket-planets"  and  the 
huge  suns  and  worlds  with  which  we  are  distantly  acquainted. 

In  some  places  these  airy  particles  of  sand  and  gravel  are 
thousands  of  miles  apart,  but  sometimes  they  are  so  crowded 
together  that  they  are  only  twenty  or  thirty  miles  from  one 
another. 

As  these  fragments  of  floating  rock  are  familiar  in  appearance, 
small  in  size,  non-aggressive  in  disposition,  and  sedentary  in 
habit,  we  will  not  concern  ourselves  any  more  with  them  at  pres- 
ent, but  will  stop  in  one  place  and  sweep  the  neighbourhood  with 
a  searchlight  to  see  if  anything  more  important  passes  our  way. 


THE  MESSENGERS   OF  HEAVEN 

We  may  have  to  watch  for  what  seems  to  us  a  big  part  of 
eternity,  or  our  search  may  be  successful  before  an  earthly  year 
has  passed  away. 

At  last  our  patience  is  rewarded  by  seeing  something  coming 
in  our  direction.  It  is  approaching  so  leisurely  that  we  have 
plenty  of  time  to  observe  it,  as  it  almost  imperceptibly  drifts  by 
our  lonely  station. 

With  the  help  of  suitable  instruments  we  can  see  that  our 
visitor  consists  of  a  nucleus  of  fragmentary  solids  surrounded 
by  a  thin  but  misty  atmosphere  which  emits  a  faintly  phos- 
phorescent glow.  The  density  of  the  whole  mass  is  so  small 
that  the  solid  nucleus  is  apparently  in  the  form  of  dust  or  ashes, 
with  perhaps  a  nucleolus  of  larger  fragments.  All  are  practi- 
cally without  weight,  and  rest  on  one  another 
as  gently  as  so  much  feathery  down. 

Our  visitor  is  not  very  far  away  from  us,  as 
celestial  distances  go,  and  yet  it  appears  to 
be  almost  at  a  standstill.  Considering  its 
size  it  really  seems  to  be  the  quietest,  laziest, 
deadest  object  we  have  yet  come  across  in  the 
Universe.  With  the  exception  of  the  tiny  ^  99.— A  CELES- 
meteoric  stones  just  mentioned,  we  have  never  TIAL  MESSENGER  ON 
seen  its  equal  in  these  respects. 

Let  us  follow  this  strange  object  in  its  do^iVpSntta^Vei^ 
leisurely  travels  through  space.  But  be  care-  ula-  it  should  be  viewed 

„    ,   ,         J  .  .  r   .  ,  from  a  distance. 

lul  lest  your  impatience  at  its  slowness  does 
not  bring  on  that  kind  of  nervous  attack  which  is  known  by 
western  deer-hunters  as  the  "buck-ager,"  for  you  might  pos- 
sibly scare  the  lazy-looking  object,  which  is  not  as  dead  as  it 
looks,  and  is  indeed  capable  of  travelling  at  a  speed  that  would 
almost  leave  the  Chariot  of  Imagination  behind. 

After  keeping  the  new  arrival  in  view  for  a  million  years  or 
so,  it  becomes  evident  that  its  drifting  motion  is  not  due  to  any 
energy  of  its  own,  but  is  caused  by  the  gravitational  attraction 
of  the  far  distant  stars. 


HOW   TO   KNOW   THE   STARRY   HEAVENS 


KEARING  PORT 

In  the  course  of  long  and  weary  ages  the  object  we  are 
investigating  gradually  and  imperceptibly  enters  the  area 
where  the  attractive  energy  exerted  by  one  star  is  greater  than 
that  of  all  the  other  stars  put  together.  The  result  is  that  it 
now  moves  faster  than  before,  and  (like  a  messenger-boy  near- 
ing  his  destination)  begins  to  look  as  though  it  were  going 
somewhere. 

It  yields  itself  without  a  struggle  to  the  growing  influence, 
and  imperceptibly  quickens  its  speed  with  every  mile  it  goes. 
As  the  time  passes  away,  the  now  progressive  messenger  leaves 
the  cold  and  midnight  darkness  of  outer  space,  and  enters  the 
serene  twilight  that  reigns  eternal  at  the  portals  of  the  solar 
system  it  is  visiting. 

As  it  approaches  the  brilliantly  glowing  star  and  comes  within 
the  radius  of  its  circling  family  of  worlds,  it  increases  still  more 
the  rapidity  of  its  forward  movement.  Faster  and  yet  faster  it 
speeds  through  the  gathering  light.  The  intense  cold  of  inter- 
stellar space  has  now  moderated.  The  new  arrival  feels  the 
growing  warmth,  and  begins  to  unfold  itself  before  the  cheerful 
influence.  The  surrounding  haze  gradually  melts  away  into  a 
glowing  gas  which  forms  a  huge  luminous  atmosphere  around 
the  loose  and  scattering  nucleus. 

By  this  time  the  outer  members  of  the  solar  family  have  been 
safely  passed.  The  now  rushing  messenger  is  hastening  faster 
and  yet  faster  toward  the  central  star.  The  milestones  of  space 
fall  behind  as  the  pickets  of  a  fence  flicker  when  seen  close  at 
hand  from  an  express  train. 

The  so-called  Asteroids  are  passed  with  a  fearful  disregard 
for  possible  collisions.  Then,  with  still  gathering  intensity  of 
ever-increasing  speed,  our  celestial  messenger  flashes  like  a 
winged  sword  of  fire  through  the  midst  of  the  inner  planets  as 
they  circle  serenely  around  their  central  sun.  So  great  is  the 
speed  of  the  messenger's  body  that  its  flowing  robes  of  luminous 
particles  now  trail  behind  for  many  millions  of  miles.  They 


FIG.   100.  —  A  CELESTIAL  MESSENGER  APPROACHING  A  STAR 


FIG.  101.  —  BROOK'S  COMET,   1893 

Lick  photograph.     The  tail  was  distorted,  probably 
by  collision  with  a  swarm  of  meteoric  bodies. 


FIG.   102.  —  COMET  1903   C 
Lick  photograph. 


THE  MESSENGERS   OF   HEAVEN  223 

look,  indeed,  like   the  "  wake  "  of   a   fast-moving   steamer  on 
smooth  water  (see  Figure  100). 

A  CELESTIAL  MESSENGER  APPROACHING  A  STAR 

The  inhabitants  of  the  minor  planets  tremble  by  night  as 
they  see  what  looks  like  a  mighty  two-edged  sword  of  fire 
flaming  in  the  starlit  sky.  Some  of  the  more  ignorant  of 
them  pray  to  their  Gods  to  deliver  them  from  the  threatening 
monster.  Others,  with  more  self-reliance,  beat  their  tom-toms 
to  scare  away  the  intruder  from  their  neighbourhood.  But,  all 
unconscious  of  the  commotion  it  has  caused  among  the  self- 
important  microbes  that  cling  around  the  circling  cobble-stones, 
the  mighty  visitor  sweeps  majestically  by  on  its  appointed 
path,  leading  to  the  central  star. 

A  HOT  RECEPTION 

At  last,  with  a  still  more  energetic  rush  that  almost  rivals 
the  flight  of  the  swift- winged  arrows  of  light,  the  messenger  of 
the  Gods  swoops  down  —  down  —  down,  and  is  lost  to  sight 
in  the  blasting  heat  and  dazzling  light  of  the  solar  furnaces. 

To  all  appearances  our  messenger  of  the  Gods  has  met  with 
such  a  fiery  reception  that  his  reappearance  is  not  to  be  looked 
for.  His  fall  seemed  to  be  as  disastrous  as  that  of  the  fabled 
Adversary,  whom  (according  to  Milton)  — 

—  "the  Almighty  Power 

Hurled  headlong  flaming  from  the  ethereal  sky, 
With  hideous  ruin  and  combustion,  down 
To  bottomless  perdition,  there  to  dwell 
In  adamantine  chains  and  penal  fire, 
Who  durst  defy  the  Omnipotent  to  arms."  l 

One  thing,  however,  we  noticed  as  our  celestial  messenger 
disappeared  in  the  solar  light,  and  that  was  that  he  did  not  fall 
prone  into  the  solar  flames,  as  the  traditional  Adversary  crashed 
into  the  molten  crater  of  the  hellish  Kilauea.  He  seemed,  in- 

1  "  Paradise  Lost,"  Book  I. 


224,     HOW  TO   KNOW  THE   STARRY   HEAVENS 

deed,  to  fall  to  one  side,  as  though  he  meant  to  skim  along  the 
surface,  deliver  his  message,  and  rush  out  on  the  other  side 
before  the  terrific  heat  could  burn  him  up. 

Knowing,  however,  how  fearful  is  the  heat  of  that  stu- 
pendous globular  furnace,  it  does  not  seem  worth  while  staying 
to  see  if  he  has  survived  the  ordeal.  Indeed,  it  is  uncomfort- 
ably hot  even  fifty  millions  of  miles  away  from  the  blazing 
star.  We  shall  have  to  back  out  of  the  stellar  heat  lest  our 
Chariot  of  Imagination  should  catch  fire,  or  we  should  lose 
control  of  the  horses  thereof  and  share  the  fate  of  the  presump- 
tuous Phaethon  when  he  tried  to  drive  the  chariot  of  the  Sun. 

ANOTHER  VOYAGE 

But  stay !  Something  has  just  come  into  sight  on  the  other 
side  of  the  star.  What  is  it  ?  Surely  it  cannot  be  that  our 
messenger  has  safely  skimmed  the  vast  and  fiery  furnace,  and 
has  already  reappeared  on  the  other  side,  a  million  miles  away 
from  where  he  disappeared  but  an  hour  ago !  It  seems  im- 
possible, and  yet  —  it  is  evidently  the  same  messenger,  hasten- 
ing away  in  safety  from  the  fiery  ordeal  through  which  he  has 
passed. 

So  far  from  being  injured  by  the  encounter,  he  (like  the 
grand  hero  of  "  Paradise  Lost "),  — 

"  With  fresh  alacrity  and  force  renewed, 
Springs  upward,  like  a  pyramid  of  fire, 
Into  the  wild  expanse,  and  through  the  shock 
Of  fighting  elements,  on  all  sides  round 
Environed,  wins  his  way ;  harder  beset 
And  more  endanger'd  than  when  Argo  passed 
Through  Bosphorus,  betwixt  the  justling  rocks; 
Or  when  Ulysses  on  the  larboard  shunn'd 
Charybdis,  and  by  the  other  whirlpool  steered."1 

Strange  to  relate,  our  fast-retreating  messenger  has  been 
beaten  in  the  race  by  his  flowing  garments.  These,  instead  of 
trailing  behind  in  the  solar  furnaces,  are  now  extended  in  front 

1  "  Paradise  Lost,"  Book  II. 


THE   MESSENGERS   OF  HEAVEN  225 

of  him  for  many  a  million  miles,  as  though  anxious  to  get 
back  into  the  outer  cold  and  darkness. 

Let  us  turn  our  chariot  and  trace  him  as  he  recedes  from  the 
star.  We  have  not  followed  him  very  far  before  we  find  that 
his  speed  begins  to  slacken,  and  that  he  is  apparently  gathering 
in  his  garments.  By  the  time  that  we  are  once  more  in  the 
dark,  outside  the  stellar  system,  he  is  again  nothing  but  a 
round  mass  of  faintly  glowing  haze,  drifting  idly  through  the 
midnight  sky. 

So  he  keeps  on,  century  after  century,  drifting  —  drifting  — 
drifting,  like  a  ship  becalmed  by  night  in  the  solitude  of  a 
tropic  sea. 

On  our  little  Earth  —  and  perhaps  on  millions  of  similar 
worlds  scattered  through  space  —  continents  appear  above  the 
seas  and  are  pounded  to  pieces  by  the  waves.  New  and  higher 
forms  of  life  develop,  and  give  way  in  their  turn  to  still  more 
specialised  species.  The  huge  reptiles  of  the  Earth's  Oolite 
yield  to  the  warm-blooded  mammalia  of  the  Tertiary  period. 
The  four-footed  tree-dwellers  develop  ape-like  peculiarities. 
The  sharpest  of  the  four-handed  apes  desert  the  trees  and  adapt 
themselves  once  more  to  live  on  the  ground.  Pithecanthropus 
erectus  develops  into  Palaeolithic  Man.  Neolithic  Man  is 
driven  into  the  mountains  by  those  who  have  chariots  of  iron. 
Nations,  empires,  and  races  come  into  existence,  nourish  for  a 
time,  and  pass  away,  like  ephemeric  clouds  in  a  summer  sky. 
The  chattel-slave  and  his  master  develop  into  the  serf  and  his 
lord.  The  wage-slave  and  his  "  boss  "  become  economic  equals 
in  the  Industrial  Commonwealth.  And  still  this  hazy  mass  is 
drifting  —  drifting  —  drifting,  through  ethereal  realms  of  star- 
lit space. 

A  million  times  our  World  goes  around  its  Sun,  and  still  our 
lazy  messenger  is  imperceptibly  drifting  through  endless  night. 
For  two  —  three  —  four  —  five  millions  of  Earth-timed  years 
he  drifts  —  drifts  —  drifts  along,  and  then,  coming  once  more 
under  the  influence  of  a  star,  he  repeats  his  former  rush,  but 
around  another  sun.  Again  he  goes  into  the  outer  darkness, 

15 


226     HOW  TO   KNOW   THE   STARRY   HEAVENS 

drifts  through  almost  endless  years,  and  again  he  rushes  around 
another  star.  And  so  he  spends  what  seems  to  us  almost  an 
eternity  of  time. 

A  CAPTIVE  MESSENGER 

Sometimes  the  influences  which  surround  a  star  cause  him 
to  close  his  orbit  and  circle  around  and  around  the  same  star  in 
a  long  narrow  orbit.  But  some  time  or  other  the  attraction  of 
a  planet  turns  him  out  of  his  closed  orbit,  and  he  once  more 
goes  off  and  drifts  —  drifts  —  drifts  through  endless  space. 

COMETS 

It  is  hardly  necessary  to  state  that  our  Messenger  of  the 
Gods  is  nothing  more  or  less  than  what  is  known  to  us  as  a 
comet.  Such  bodies  exist  throughout  the  Universe  in  hundreds 
of  millions.  Until  recently  we  were  entirely  in  the  dark  as  to 
their  composition,  origin,  and  movements.  Their  nuclei,  it  is 
true,  were  known  to  submit  to  the  law  of  gravitation,  but 
hardly  a  guess  could  be  made  as  to  the  nature  of  the  repulsive 
force  which  produces  and  dominates  over  their  tails.  Even  yet 
we  have  much  to  learn  concerning  comets  generally.  But  some 
of  the  main  facts  have  been  ascertained,  and  a  reasonable 
theory  has  been  formed  which  throws  a  good  deal  of  light 
upon  them. 

The  probability  is  that  they  are  huge  collections  of  loose 
meteoric  dust  and  gravel,  with  large  quantities  of  hydrocarbons 
and  free  hydrogen.  All  this  loose  material  has  been  ejected 
into  space  from  solar  or  planetary  volcanoes.  In  the  intense 
cold  of  outer  space  (which  is  at  least  230°  below  zero  on  the 
Fahrenheit  scale,  and  may  possibly  be  —  461°),  the  hydrogen 
and  hydrocarbons  naturally  cling  evenly  around  the  solider 
material. 

On  approaching  a  star,  the  radiant  energy  from  the  central 
sun  causes  them  to  spread  in  all  directions  around  the  scatter- 
ing nucleus.  They  thus  form  the  cloudy  haze  of  which  all 


THE   MESSENGERS   OF   HEAVEN  227 

telescopic  comets  consist.  When  still  nearer,  the  spreading 
particles  in  front  are  further  dissipated  by  the  heat.  At  last 
they  are  so  small  that  the  radiant  energy  of  the  sun  overcomes 
their  gravity,  and  violently  repels  them  into  outer  space. 
There,  lighted  up  by  the  sun,  and  glowing  with  a  soft  electric 
light  of  their  own,  they  form  the  tremendous  hollow  luminous 


FIG.  103.  —  PARABOLIC  ORBIT  OF  A  FREE  COMET 

It  is  practically  an  ellipse  with  the  two  foci  at  an  infinite 
distance  from  each  other.  Hence  the  orbit  is  closed  at  only 
one  end. 

trumpet-shaped  appendage  known  as  the  tail  of  the  comet. 
The  shape  of  the  tail,  and  its  direction  in  space,  depend  on  the 
strength  of  the  repelling  force,  and  that  varies  with  the  size  of 
the  particles  composing  the  tail.  Sometimes  there  are  several 
tails,  extending  in  different  directions,  or,  rather,  in  different 
curves.  There  appear  to  be  three  different  types  of  tail,  differ- 
ing in  direction,  shape,  and  material.  The  straightest  ones  are 
probably  composed  of  hydrogen.  They  are  repelled  with  the 
greatest  force,  and  therefore  extend  almost  directly  away  from 
the  Sun.  The  next  are  more  curved,  and  are  supposed  to  con- 


228    HOW  TO   KNOW  THE   STARRY   HEAVENS 

sist  of  varying  hydrocarbons.  They  are  repelled  with  about 
one  quarter  the  force  of  the  hydrogen  tail.  The  third  type  is 
still  more  curved,  and  probably  consists  of  chlorine  and  iron. 
The  repulsion  is  only  about  one  sixteenth  that  of  the  hydrogen 
tail.  All  of  these  tails  are  hollow  cones,  of  which  only  the 
sides  are  generally  visible. 


FIG.  104.  —  ELLIPTICAL  ORBITS  OF  CAPTIVE  COMETS 

As  fresh  particles  are  being  sent  out  into  each  tail  all  the 
time,  it  naturally  follows  that  after  the  head  of  the  comet  has 
begun  to  recede  from  the  Sun  its  newly  formed  tails  stretch  out 
in  front  of  it.  In  the  meantime  the  old  ones  have  been  driven 
off  and  dispersed. 

As  before  stated,  the  main  body  of  a  comet  obeys  the  same 
laws  that  control  the  motions  of  all  the  heavenly  bodies.  The 
luminous  gas  of  which  a  comet  is  largely  composed  is  so  thin 
and  transparent  when  within  the  Solar  System  that  the  faintest 
stars  can  be  seen  through  millions  of  miles  of  its  substance. 
The  whole  comet  is  so  light  in  weight  that  it  can  sweep  by  the 


FIG.   106.  —  DONATI'S  COMET,  1858 
By  Bond.     (From  Comstock's  "  Text-book  of  Astronomy," published  by  Messrs.  D.  Appleton  &  Co.) 


FIG.   107.  —  COMET   RORDAME,    1893 
By  Hussey,  at  Lick  Observatory. 


THE   MESSENGERS   OF  HEAVEN 

planet  Saturn  without  disturbing  the  motions  of  his  numerous 
family  of  moons,  while  these  same  moons,  small  as  they  are, 
are  able  to  deflect  it  from  its  original  path  if  it  should  happen 
to  pass  near  them. 

PERIODIC  COMETS 

"  Amid  the  radiant  orbs 
That  more  than  deck,  that  animate  the  sky, 
The  life-infusing  suns  of  other  wcfrlds ; 
Lo !  from  the  dread  immensity  of  space, 
Returning  with  accelerated  course, 
The  rushing  comet  to  the  Sun  descends ; 
And  as  he  sinks  below  the  shading  Earth, 
With  awful  train  projected  o'er  the  heavens, 
The  guilty  nations  tremble." —  Thomson,  "The  Seasons" — Summer. 

Some  of  the  comets  that  move  in  closed  orbits  around  our 
Sun  have  had  their  elliptical  paths  traced  out  by  the  astron- 
omers.    They  are  not  confined  to  the  Ecliptic,  like  the  planets, 
but  are  inclined  at  all  angles 
to  it.    Their  regular  coming  can 
be    anticipated,    though    their 
movements  are  never  to  be  de- 
pended on,  owing  to  the  ease 
with    which   they    are    turned 
out  of  their  course  by  the  plan- 
ets they  happen  to  come  near. 

Several    of     the     periodical 

comets  have   paths   extending 

,,          ,  .,      „  \      .  _.      6      FIG.  105.  —  TAIL  OF  A  COMET  NEAR 

to  the  orbit  of  Jupiter.    Others  PERIHELION 

go  away  as  far  as  Saturn  and 

Uranus.  This  is  one  of  the  peculiarities  which  gave  rise  to 
the  theory  that  they  were  originally  thrown  out  from  those 
giant  planets  by  tremendous  volcanic  eruptions,  like  those 
which  are  constantly  taking  place  on  the  apparent  surface  of 
our  Sun.  This  theory  is  now  generally  abandoned,  on  account 
of  the  tremendous  force  necessary  to  overcome  the  gravitation 
of  such  huge  planets. 

They  are  now  believed  to  have  been  captured  by  the  planets 


230     HOW   TO   KNOW   THE   STARRY   HEAVENS 

to  which  they  are  related.  Tempel's  comet,  which  is  connected 
with  the  Leonid  meteors  —  and  which  goes  away  as  far  as  the 
orbit  of  Uranus  —  has  had  its  orbit  traced  back  to  the  year 
126  A.  D.  At  that  date  the  two  bodies  were  very  near  together, 
and  the  planet  appears  to  have  drawn  it  (and  the  Leonids 
accompanying  it),  out  of  the  original  parabolic  orbit  into  an 
elliptical  orbit  having  a  period  of  about  33  years.  Such 
changes  of  orbit  appear  to  be  not  uncommon  among  comets. 

The  spectra  of  comets  .contain  three  bright  hydrocarbon 
bands  well  denned  at  the  red  end.  There  are  also  several 
bright  iron  lines,  and  one  due  to  manganese.  When  a  comet  is 
near  the  Sun,  the  bright  sodium  lines  are  sometimes  seen.  In 
addition  to  this  emission  or  radiation  spectrum,  there  is 
generally  a  faint  absorption  spectrum  visible,  with  dark  lines. 

METEORS 

Another  peculiarity  of  these  erratic  wanderers  is  that  some 
of  them  are  intimately  connected  with  meteor  showers.  When 


FIG.  108.  —  A  METEOR  BURSTING  IN  THE  ATMOSPHERE 

Seen  through  a  telescope. 

our  Earth  passes  through  the  path  of  one  of  these  captive 
comets,  vast  multitudes  of  so-called  "  shooting-stars  "  enter  our 
atmosphere  and  burn  up  with  the  heat  generated  by  atmospheric 
friction.  Where  fragments  of  meteors  happen  to  reach  the. 


THE   MESSENGERS   OF  HEAVEN  231 

surface  of  the  Earth,  they  are  found  to  consist  of  volcanic  sub- 
stances like  those  thrown  out  by  earthly  volcanoes.  Many  of 
them  consist  of  irregular  fragments  cemented  together.  Some 
contain  considerable  iron,  and  carbon  has  been  found  in  a  few. 
Even  mineral  veins  have  been  found  in  them. 

Our  Earth  has  now  lost  a  great  part  of  its  volcanic  activity, 
yet  the  eruption  of  Krakatoa,  a  few  years  ago,  was  almost 
powerful  enough  to  cast  its  debris  clear  of  the  planet's 
attraction. 

Supposing  such  a  thing  to  take  place,  the  cast-out  masses, 
both  gaseous  and  solid,  would  naturally  turn  toward  the  Sun, 
rush  around  that  mighty  globe,  and,  if  not  consumed  by  its 
heat,  return  to  the  place  where  the  Earth  was  when  they  were 
ejected.  Not  finding  the  planet  where  it  had  been,  the  stream 
of  meteoric  matter  would  naturally  turn  to  the  Sun  again,  and 
continue  in  the  same  orbit  until  its  Earth-derived  particles  were 
all  picked  up  again  by  the  Earth  or  Moon. 

If  these  comets  and  clouds  of  meteoric  dust  should  originate 
in  the  way  described,  it  follows  that  moderate-sized  worlds  of 
other  systems  must  be  equally  capable  of  casting  out  similar 
masses  of  cometary  gas  and  meteoric  solids.  This  would  explain 
the  great  abundance  of  comets  and  meteoric  streams  in  our  own 
system. 

There  is  some  evidence  that  both  comets  and  meteoric  streams 
gradually  waste  away  when  travelling  through  solar  systems. 
At  each  approach  to  a  star,  a  comet  appears  to  lose  the  repelled 
matter  which  forms  its  dissipated  tail  or  tails.  After  a  certain 
number  of  perihelia  the  supply  of  hydrogen  and  hydrocarbons 
fails.  Short-period  comets  are  therefore  generally  tailless.  A 
comet  has  also  been  known  to  split  up  into  two  separate  bodies 
which  gradually  drifted  apart.  In  some  cometary  orbits  there 
appear  to  be  several  comets,  all  moving  in  the  same  path,  but 
at  long  distances  apart.  Sometimes  the  solid  bodies  forming 
the  nucleus  of  a  comet  appear  to  spread  out  along  its  orbit  until 
the  comet  disappears  and  nothing  remains  but  a  long  trail  of 
invisible  meteoric  matter.  Meteoric  streams  also  waste  away 


232    HOW  TO   KNOW  THE   STARRY   HEAVENS 

to  some  extent,  owing  to  the  capture  or  deflection  of  fragments 
by  passing  planets.  The  captured  fragments  of  course  form  the 
meteors  and  "  shooting-stars  "  with  which  we  are  so  familiar  on 
Planet  Number  Three. 

It  seems,  therefore,  that,  long-lived  as  the  comets  are,  they 
too  have  a  limit  in  duration.  They  are  not  "  built  for  eternity," 
but  are  as  ephemeral  as  the  suns  and  worlds  among  which  they 
wander.  But  the  loss  is  evidently  made  up  by  the  constant 
formation  of  fresh  comets  and  meteoric  streams  in  various  parts 
of  the  Universe. 


CHAPTER  XX 

LARGE  AND  SMALL  WORLDS 

"  There  are  worlds  so  vast  that  beside  them  our  Earth  would  seem  but  a  toy. 
There  are  worlds  so  small  that  they  might  serve  as  marbles  for  our  children  to 
play  with."  —  A.  Zazd. 

IT  has  been  already  shown  that  both  suns  and  worlds  vary 
enormously  in  size  and  in  the  amount  of  matter  they  con- 
tain. But  it  is  very  hard  to  realise  how  vast  these  differences 
are.  In  our  own  System,  while  some  planets  are  immensely 
larger  than  our  Earth,  others  are  too  small  to  be  visible  with 
any  telescope  unless  they  happen  to  be  massed  together  in  mil- 
lions, as  in  the  rings  of  Saturn.  It  may  be  well  to  illustrate 
some  of  these  differences,  paying  special  attention  to  the  Third 
Planet,  the  largest  planet,  and  the  Sun. 

MICROSCOPIC  WORLDS 

There  are  "  planets  "  going  around  the  Sun  which  are  so  small 
that  if  we  had  them  in  our  hands  we  could  not  see  them  with- 
out a  microscope.  They  are  commonly  called  meteoric  bodies, 
yet  they  really  and  truly  are  worlds  like  ours,  revolving  in  or- 
bits more  or  less  similar  to  ours.  They  are  subject  to  the  same 
laws,  and  may  have  had  a  separate  existence  as  long  as  the 
Earth  on  which  we  live. 

At  the  same  time  there  are  planets,  going  around  the  same 
Sun,  so  vast  that  our  Earth  would  appear  utterly  insignificant 
beside  them. 

OUR  INSIGNIFICANT  EARTH 

Jupiter,  the  largest  planet  in  our  system,  weighs  300  times 
as  much  as  our  Earth.  Not  having  yet  cooled  down  so  much 
as  the  Earth,  it  is  more  than  1,200  times  as  large. 


234     HOW  TO   KNOW  THE   STARRY  HEAVENS 

Our  Sun  weighs  330,000  times  as  much  as  the  Earth,  and 
for  the  same  reason  is  1,250,000  times  as  large. 

Two  or  three  illustrations  may  make  these  comparisons 
plainer. 

If  our  Earth  be  represented  by  a  ball  one  inch  in  diameter, 
Jupiter's  weight  will  be  represented  by  a  globe,  of  the  same 


SATURN. 


*•»«  ©          9  f|  @  6 

ASTEROIDS.      MARS.  MOON.    EARTH.    VENUS.   MERCURY. 


FIG.  110.  —  RELATIVE  SIZES  OF  PLANETS 

density,  4.2  inches  thick,  and  that  of  the  Sun  by  a  globe  5  feet 
9  inches  across. 

With  the  same  one-inch  ball  for  our  Earth,  Jupiter's  size  will 
be  represented  by  a  globe  11  inches  thick,  and  that  of  the  Sun 
by  a  globe  9  feet  across. 

If  we  were  to  place  in  a  straight  line  enough  of  these  one- 
inch  globes  to  represent  the  weight  of  Jupiter,  they  would 
extend  25  feet,  while  if  we  were  to  put  in  a  line  enough  to 
represent  the  size  of  Jupiter,  they  would  reach  100  feet. 

To  represent  the  Sun's  weight  in  the  same  way,  we  should 


LARGE   AND   SMALL   WORLDS 


235 


reguire  a  string  of  them  more  than  five  miles  long.     And  for 
its  size,  nearly  twenty  miles  of  them. 

The  difference  between  the  mass  of  the  Sun  and  that  of  the 
Earth  may  also  be  illustrated  in  this  way.  A  weight  dropped 
from  a  height  on  our  Earth  falls  a  little  over  16  feet  in  the  first 
second.  But  on  the  Sun  it 
would  fall  452  feet  in  the 
same  interval  of  time.  An  ob- 
ject which,  on  account  of  the 
Earth's  attraction,  here 
weighs  one  pound,  if  removed 
to  the  apparent  surface  of 
our  Sun,  and  weighed  by  a 
spring  balance,  would  be 
found  to  have  increased  its 
weight  to  28  pounds  on  ac- 
count of  the  greater  mass 
and  attraction  of  the  Sun. 

These  two  illustrations  do 
not  really  do  justice  to  the 
difference  between  the  mass 
of  the  Earth  and  that  of  the 
Sun.  For  the  Sun's  centre  of 
attraction  is  108  times  as  far 
from  the  object  on  its  surface 
as  in  the  case  of  the  Earth. 

To  get  over  this  difficulty,  let  us  imagine  the  Sun  to  be  com- 
pressed until  it  is  the  same  size  as  the  Earth,  without  dimin- 
ishing its  mass.  It  will  then  be  330,000  times  as  dense  as  the 
Earth.  The  object  which,  on  the  Earth,  weighed  a  pound,  will 
now,  when  removed  to  this  compressed  Sun,  be  found  to  weigh 
165  American  tons.  And  a  weight  dropped  from  a  height  will 
fall  1,000  miles  in  the  first  second.1 

1  This  compressed  illustration  is  of  course  impossible,  but  it  will  serve  to  show 
the  immense  difference  in  the  quantity  of  material  contained  in  the  Earth  and 
Suu. 


FIG.  111.  — RELATIVE  SIZES  OF  SUN, 
JUPITER,  AND  EARTH 


236     HOW  TO   KNOW  THE   STARRY   HEAVENS 


All  the  other  planets  in  our  System,  taken  together,  weigh 
about  450  times  as  much  as  our  Earth,  yet  the  Sun  outweighs 
them  all  745  times.  If  all  the  planets  be  represented  by  a 
heap  of  boulders  weighing  450  pounds,  a  little  one-pound  pebble 
will  stand  for  the  Earth.  And  on  the  same  scale  the  Sun  will 
be  represented  by  a  mighty  rock  weighing  about  170  American 
tons. 

Our  Moon  is  2,162  miles  in  diameter,  and  looks,  from  here, 
as  though  it  were  the  same  size  as  the  Sun,  yet  the  latter  ex- 
ceeds   it    in    bulk    more  than 
60,000,000  times,  and  in  weight 
27,000,000  times. 

Go  out  on  an  unusually  clear 
night,  when  the  Moon  is  below 
the  horizon.  Notice  what  a 
multitude  of  stars  can  be  seen 
by  the  naked  eye  alone.  Imag- 
ine every  star  visible  to  be  600 
times  as  large  as  our  Earth. 
Then  all  of  them  rolled  together 
into  one  vast  globe  would  not 
be  as  large  as  the  Sun. 

Yet  our  Sun  is  comparatively 
a  small  star.  Canopus,  one  of 
the  bright  stars  in  the  southern 
hemisphere,  gives  out  many 

thousands  of  times  as  much  light.     If  its  lustre  is  equal  to  that 
of  the  Sun,  it  must  be  many  thousands  of  times  larger. 


FIG.  112.  —  RELATIVE  SIZES  OF  THE 
FIRST  FOUK  ASTEROIDS  AND  THE 
EARTH'S  SATELLITE.  BY  BARNARD 

The  black  circle  represents  the  Moon, 
and  the  white  ones  the  Asteroids. 


OUR  MIGHTY  GLOBE 


We  thus  see  that  our  planet  is  a  very  insignificant  world 
compared  with  some  of  the  other  heavenly  bodies.  Yet  it  con- 
tains what  seems  to  us  to  be  a  considerable  amount  of  material. 

Let  us  suppose  the  Earth  to  be  drawn  out  into  a  long  square 
column  a  mile  thick  each  way.  This  column  would  reach  from 


LARGE   AND  SMALL  WORLDS  237 

the  Sun  to  a  distance  93  times  as  great  as  that  of  the  planet 
Neptune. 

Let  us  suppose  that  this  long  column  was  put  on  suitable 
flat-cars  and  started  off  at  50  miles  an  hour.  Then  suppose  we 
saw  it  coming,  and  decided  not  to  cross  the  track  till  it  had 
passed  by.  We  should  have  to  wait  590,000  years  before  'the 
line  would  be  clear. 

A  signal-light  flashed  from  one  end  of  this  train,  at  the  usual 
speed  of  186,000  miles  in  a  second  of  time,  would  not  be  seen 
at  the  other  end  for  sixteen  days. 

If  our  Earth  could  be  crushed  under  foot  till  it  was  flattened 
out,  so  as  to  be  only  a  mile  thick,  it  would  form  a  round  flat 
disc,  565,000  miles  across,  —  considerably  larger  than  the 
Moon's  orbit. 

So  that  if  our  Earth  is  of  but  slight  importance  in  the  Uni- 
verse, it  must  be  admitted  that  it  is  a  considerable  size  when 
looked  at  from  a  human  standpoint. 

THE  CRASH  OF  WORLDS 

Theoretically,  every  atom  of  matter  in  the  Universe  influences 
every  other  atom  of  matter  in  the  Universe.  And  practically 
it  influences  all  within  x  millions  of  miles. 

Yet  as  a  rule  the  myriads  of  suns  and  worlds  do  not  interfere 
with  one  another.  They  keep  on  in  the  even  tenor  of  their 
way  from  century  to  century.  For  many  ten  thousands  of 
millenniums  they  keep  out  of  one  another's  way. 

Still,  accidents  will  happen  even  among  solar  systems. 
There  appear  to  be  times  when  suns  and  planets  come  into  col- 
lision, when  even  worlds  suffer  shipwreck. 

Some  of  the  stellar  systems  are  drifting  along  at  the  rate  of 
two  hundred  miles  in  one  second  of  time.  Therefore  such  catas- 
trophes are  liable  to  give  rise  to  a  considerable  amount  of  light 
and  heat. 

Whenever  you  see  what  is  miscalled  a  shooting -star  dart 
across  the  sky  and  disappear,  you  witness  the  destruction  of  a 
"  pocket-planet  "  which  may  be  as  old  as  our  Earth. 


238    HOW  TO   KNOW  THE   STARRY  HEAVENS 

Whenever  you  see  the  fiery  rush  of  a  meteor,  and  hear  its 
distant  crash,  you  may  know  that  another  little  world  has  met 
its  doom  and  ceased  to  have  an  independent  existence. 

Whenever  a  star  suddenly  increases  in  brilliancy,  and  for  a 
time  gives  out  many  thousands  of  times  its  former  light,  you 
may  feel  tolerably  certain  that  mighty  suns  have  crashed  into 
mutual  destruction. 

Yet  nothing  is  really  destroyed.  The  "  shooting-star  "  still 
exists  as  vapour  or  dust  in  our  atmosphere ;  the  meteor  settles 
down  to  form  part  of  oUr  Earth;  the  crashing  suns,  though 
turned  to  naming  gas,  unite  and  begin  once  more  the  same  end- 
less cycle  of  evolution  and  devolution.  There  is  no  end,  nor 
was  there  yet  beginning. 

STARS  ARE  SOLITARY 

"  His  soul  was  like  a  star,  and  dwelt  apart." 

Many  people  still  have  the  old  idea  that  the  stars,  although 
a  long  way  from  us,  are  comparatively  close  to  one  another. 
This  is  not  at  all  correct,  in  spite  of  the  fact  that  in  thousands 
of  cases  two,  three,  or  more  stars  form  one  system. 

Some  of  the  star-clusters  appear  to  be  democratic  systems 
composed  of  vast  numbers  of  comparatively  small  suns  revolv- 
ing around  their  common  centres  of  gravity.  Thus  the  cluster 
Omega  Centauri  consists  of  more  than  6,000  stars,  of  which  at 
least  125  are  variables  (see  Figure  94).  In  cases  like  this  it  is 
not  likely  that  the  individual  suns  have  planets  revolving 
around  them,  or,  at  least,  not  habitable  ones. 

But  leaving  these  multiple  systems  out  of  consideration,  it 
may  be  said  that  each  star  is,  with  the  exception  of  its  subject 
worlds,  solitary  in  space.  If  we  could  take  our  stand  at  a  com- 
fortable distance  from  any  one  of  these  sovereign  stars,  we 
should  imagine  the  system  to  be  in  the  centre  of  a  huge  void, 
surrounded,  at  an  enormous  distance,  by  a  hollow  sphere  of 
crowded  stars.  Or,  in  other  words,  the  heavens  would  look 
about  the  same  as  they  do  from  our  Solar  System.  In  the  case 


LARGE   AND   SMALL  WORLDS  239 

of  a  neighbouring  star,  only  an  expert  could  detect  any  changes 
in  the  constellations. 

When  we  see  two  stars  which  appear  to  be  near  to  each 
other,  we  must  remember  that  in  many  cases  one  star  is  two, 
ten,  twenty,  or  a  hundred  times  as  far  from  us  as  the  other  one. 
It  may  even  be  that  the  brighter  one  is  the  most  distant. 

On  this  account  the  apparent  brightness  of  a  star  as  seen  from 
our  Earth  is  a  very  unreliable  guide  as  to  its  distance  or  size. 
As  a  matter  of  fact,  no  two  stars  are  alike  in  actual  size,  bril- 
liancy, colour,  or  any  other  peculiarity. 

STABS  ARE  MIGHTY  SUNS 

"  The  stars  of  heaven  fell  to  the  ground,  as  green  figs  fall  jvhen  the  tree  is 
shaken  by  a  mighty  wind."  —  Rev.  vi,  15. 

Then  we  must  remember  that  every  one  of  these  sovereign 
stars  is  a  SUN  more  or  less  like  our  Sun.  Every  one  seen  is  at 
least  something  like  1,000,000  times  as  large  as  our  Earth. 
The  smallest  visible  is  worthy  of  the  name  of  SUN  ;  is  large 
enough,  powerful  enough,  hot  enough,  and  bright  enough,  to 
hold  sway  over  worlds  as  beautiful  as  Venus,  as  fiery  as  Mars, 
as  vast  as  Jupiter,  as  magnificent  as  Saturn,  as  distant  as  Nep- 
tune, and  as  populous  as  the  little  Earth  on  which  we  live. 

Every  star  visible  is  blazing  with  a  light  peculiar  to  itself, 
different  from  the  light  of  any  other.  In  the  vast  majority  of 
cases  this  light  is  intense  enough  to  make  the  Columbian 
search-light  look  black  by  comparison. 

Each  of  these  suns  is  throbbing  with  quaking  blasts  of  fer- 
vent heat  that  would  make  the  molten  interior  of  a  Bessemer 
converter  seem  cold  by  comparison.  The  eruptions  of  Mont 
Pe'le'e,  fearful  as  they  seemed  to  us,  were  but  the  sputterings  of 
a  bunch  of  Fourth  of  July  fire-crackers  when  compared  with  the 
awful  cannonading  which  goes  on,  every  day  in  the  year,  all 
around  the  average  star. 

So  intense  is  the  heat  of  our  own  central  star  that  if  the 
Earth  were  to  be  checked  in  its  career  and  left  to  fall  toward 


240     HOW  TO   KNOW   THE   STARRY  HEAVENS 

the  Sun,  it  would  never  reach  the  photosphere  in  a  solid  state, 
but  would  be  turned  into  flaming  gas,  and  driven  off  again,  like 
a  hailstone  falling  toward  a  mass  of  molten  steel.  Sir  John 
Herschel  showed  that  if  an  icicle  45  miles  in  diameter  could 
be  driven  endways  into  the  Sun  with  the  velocity  of  light 
(186,000  miles  per  second),  it  would  be  melted  off  as  quickly  as 
it  advanced.  Some  of  the  larger  stars  could  dispose  of  an  icicle 
more  than  100  miles  in  diameter. 

Every  star  is  continuously  roaring  and  throbbing  with  ear- 
rending  detonations  and  world-jarring  convulsions  that  are 
greater,  in  one  short  hour,  than  all  our  thunder  and  lightning, 
earthquakes  and  volcanic  eruptions,  for  millions  of  years  gone 
by.  Every  one  is  dragging  its  attendant  family  of  worlds 
through  the  wilderness  of  space  at  a  speed  that  the  mind  of 
man  cannot  realise,  it  is  so  tremendous. 

A  TINY  GLOBE 

"The  Earth  is  my  foot-stool."  —  (The  Later]  Isaiah,  Ixvi,  1. 

In  all  this  splendour  of  molten  orbs,  our  Earth,  fortunately 
for  us,  has  no  part.  Silent,  dark,  and  invisible  (except  to  three 
or  four  of  its  nearest  companions),  it  is  of  no  measurable  im- 
portance in  the  economy  of  Nature.  So  far  as  other  worlds  are 
concerned,  its  existence  is  no  benefit,  its  disappearance  would 
cause  no  anxiety,  its  destruction  would  be  no  loss,  its  absence 
would  give  no  trouble. 

Yet,  fastened  to  the  radiant  skirts  of  the  glorious  Sun  by 
invisible  yet  unbreakable  bonds,  it  has  been  dragged  for  many 
millions  of  years,  through  the  wilds  of  space,  at  a  speed  incon- 
ceivably great.  Small  though  it  is,  it  is  teeming  with  life  of 
every  kind.  It  contains  almost  boundless  oceans  and  con- 
tinents. It  has  mountains  and  valleys,  rivers  and  lakes,  islands 
and  seas,  beautiful  beyond  the  power  of  words  to  describe.  It 
has  a  history  extending  back  millions  upon  millions  of  years. 
It  has  a  future  almost  without  end. 


LARGE   AND   SMALL   WORLDS 


HUMAN  ACHIEVEMENTS 

So  far  I  have  considered  only  the  smallness  and  insignificance 
of  man.  But  there  is  another  side  to  the  question.  A  thing 
may  be  small  and  yet  wonderful.  The  ancestors  of  man  were 
all  Earth-born.  He  himself  is  but  the  creature  of  a  day,  and 
has  all  his  life  been  on  one  insignificant  planet  with  no  pros- 
pect of  ever  leaving  it.  Yet,  considering  his  situation,  he  has 
done  some  wonderful  things.  Leaving  out  of  consideration  all 
his  earthly  achievements,  he  has  eaten  of  the  fruit  of  the  Tree 
of  Celestial  Knowledge  to  a  far  greater  extent  than  might  have 
been  expected.  Cooped  up  in  his  invisible  cage,  he  has  con- 
templated the  visible  parts  of  the  Universe  to  such  good  effect 
that  he  has  solved  many  of  its  mysteries.  Not  satisfied  with 
the  eyes  provided  by  Nature,  he  has  constructed  artificial 
instruments  which  increase  their  light-grasping  power  more 
than  40,000  times.  With  their  help  he  is  measuring  the  depths 
of  space,  analysing  the  stars  as  though  they  were  in  his  labora- 
tory, weighing  suns  and  worlds  in  a  balance,  and  photographing 
celestial  objects  that  are  invisible  even  with  his  instruments. 
He  traces  the  wanderings  of  the  planets  both  in  the  remote 
past  and  the  distant  future.  He  is  finding  out  the  life-history 
of  a  sun  from  its  cradle  to  its  grave.  He  watches  the  stars  so 
closely  that  millions  of  them  are  unable  to  move  without  his 
knowledge.  The  infinitely  great  and  the  infinitely  small  are 
alike  compelled  to  submit  to  his  scrutiny.  Slowly  but  surely 
he  is  compelling  Nature  to  give  up  her  innermost  secrets.  He 
is  gradually  solving  the  mystic  riddle  of  the  Universe. 

These  be  no  light  achievements  for  man  to  have  even  par- 
tially accomplished.  The  Gods  of  Greece  and  Eome  never 
attempted  such  tasks.  Odin  and  Thor  never  dreamed  of  such 
vast  undertakings.  The  labours  of  Heracles  and  feats  of  Shem- 
ishon  cannot  be  mentioned  on  the  same  page  without  provoking 
a  smile.  The  actual  achievements  of  man  surpass  those 
fabulously  attributed  to  the  Gods  and  heroes  of  antiquity. 

16 


HOW   TO   KNOW   THE   STARRY   HEAVENS 


TWO  STANDPOINTS 

"  The  Hebrews  looked  upon  themselves  as  Yahveh's  Peculiar  People,  which 
they  probably  were. 

"  The  Chinese  regard  the  '  Celestial  Empire '  as  the  most  important  part  of  the 
World,  which  it  is  —  to  them. 

"  Every  little  Boston  is  thought,  by  its  own  people,  to  be  the  '  Hub  of  the  Uni- 
verse/ which  it  possibly  may  be  —  to  those  that  dwell  therein. 

"But  a  Citizen  of  the  GREAT  COSMOS  —  though  compelled  to  view  all  things 
from  his  own  physical  standpoint  —  can  see  them  also  from  a  spiritual  standpoint 
which  is  Universal  and  Eternal."  —  A.  Zazel. 

To  the  ancients,  the  World  was  visible  only  from  a  local 
standpoint,  with  the  eyes  of  an  epheineron.  From  their  posi- 
tion it  appeared  to  be  vast  beyond  comparison,  fixed  beyond  the 
possibility  of  removal,  changeless  as  the  decrees  of  destiny, 
eternal  as  time  itself. 

We,  who  have  eaten  of  the  fruit  of  the  Tree  of  Knowledge, 
have  acquired  the  power  of  observing  the  Earth  from  both  a 
local  and  a  universal  standpoint.  We  have  learned  of  its 
insignificance ,  yet  we  better  comprehend  its  vastness.  We 
know  that  it  is  ever  on  the  wing,  yet  we  understand  the  un- 
deviating  fixity  of  the  paths  in  which  it  travels.  We  watch  its 
lands  and  seas  come  and  go  like  drifting  clouds,  yet  we  are 
aware  of  the  changelessness  of  the  laws  to  which  those  changes 
are  due.  We  know  it  as  a  transient  bubble  on  the  River  of 
Eternity,  yet  we  have  discovered  a  history  that  staggers  even 
the  imagination  itself,  and  can  foresee  a  future  that  shall  rival 
its  past. 


CHAPTER  XXI 

IGNEOUS  FORCES  ON  THE  MOON  AND  ELSEWHERE 

"  He  scarce  had  ceased,  when  the  superior  fiend 
Was  moving  toward  the  shore  :  his  ponderous  shield 
Ethereal  temper,  massy,  large,  and  round, 
Behind  him  cast ;  the  broad  circumference 
Hung  on  his  shoulders  like  the  Moon,  whose  orb 
Through  optic  glass  the  Tuscan  artist  views 
At  evening  from  the  top  of  Fesole', 
Or  in  Val  d'  Arno,  to  descry  new  lands, 
Rivers,  or  mountains,  in  her  spotty  globe." 

—  Milton,  "  Paradise  Lost,"  Bk.  I. 

BY  this  time  we  should  have  a  very  fair  idea  of  the  construc- 
tion, dimensions,  distances,  and  histories  of  the  various 
celestial  mansions  which  compose  the  visible  part  of  the  Grand 
Temple  of  the  Universe.  It  is  now  time  to  turn  nearer  home 
and  find  what  there  is  to  be  seen  and  learned  of  the  satellite 
which  is  such  a  faithful  attendant  on  the  Third  Planet  of  the 
Solar  System. 

Some  details  have  already  been  given  about  our  Moon,  but 
little  has  been  said  concerning  its  present  appearance  and 
condition,  or  about  the  changes  that  have  taken  place  on  it 
since  it  left  the  embrace  of  its  earthly  parent.1 

The  history  of  the  Moon,  like  the  histories  of  its  ancestors 
—  the  Earth,  the  Solar  System,  and  the  Universe  —  is  not 
recorded  in  books  that  were  written  by  eye-witnesses.  It  has 
to  be  obtained  by  carefully  observing  its  present  appearance 
and  condition,  and  then  by  reasoning  as  to  the  way  in  which 
natural  laws  (known  and  unknown)  can  have  brought  about 

1  Since  the  Moon  owes  its  separate  existence  to  the  gravitational  attraction  of 
the  Sun  on  the  body  of  the  Earth,  it  shares  with  many  of  the  heroes  of  antiquity 
the  honour  (?)  of  having  a  heavenly  father  and  an  earthly  mother. 


244     HOW  TO   KNOW  THE   STARRY   HEAVENS 

that  appearance  and  condition.  The  most  that  we  can  reason- 
ably hope  for,  under  these  circumstances,  is  to  get  a  fairly 
accurate  idea  of  the  main  sequence  of  events  without  laying 
too  much  stress  on  minor  details.  Indeed  we  must  not  be 
surprised  if  we  find  that  recognised  authorities  sometimes  differ 
with  regard  to  the  course  of  events,  as  well  as  to  the  relative 
importance  of  the  different  agencies  that  have  been  at  work 
on  it.  Past  experience  has  shown  that  incorrect  theories  will 
sooner  or  later  be  found  out,  and  replaced  by  others  that  are 
nearer  the  truth. 

LUNAR  FEATURES 

If  we  examine  the  Moon  with  the  unaided  eye,  when  our 
side  of  it  is  fully  illuminated  by  the  Sun,  we  can  see  very  little 
as  to  its  condition  except  that  it  appears  to  be  a  tolerably 
bright  round  disc  with  some  irregular  dark  patches  on  it. 
Each  of  these  dark  patches  remains  permanently  in  about  the 
same  part  of  the  disc,  and  has  been  there  since  history  began. 
It  is  therefore  evident  that  the  Moon  always  turns  the  same 
side  to  us.  The  only  considerable  change  is  in  the  direction 
from  which  it  is  illuminated  by  the  Sun  as  it  revolves  around 
the  circling  Earth. 

With  the  help  of  an  opera-glass  (also  used  at  full  moon), 
these  lunar  patches  become  very  much  plainer,  and  appear  to 
have  a  somewhat  circular  outline.  There  also  comes  into 
sight  a  star-like  series  of  bright  radiating  streaks,  having  their 
centre  near  the  south  of  the  visible  disc.  These  two  pecu- 
liarities together  give  the  full  moon  a  strong  resemblance  to  a 
peeled  orange  that  has  been  badly  bruised.1 

1  If  two  photographs  of  the  Moon  are  taken  from  rather  different  standpoints, 
and  then  combined  so  as  to  be  used  in  the  stereoscope,  this  resemblance  to  a  bruised 
orange  becomes  quite  startling.  Thus  viewed,  the  Moon  loses  the  flat  disc-like 
appearance  due  to  its  immense  distance,  and  stands  out  as  a  solid  sphere  just  as 
it  would  appear  to  a  giant  whose  eyes  were  thousands  of  miles  apart.  The  same 
principle  has  also  been  applied  to  planets  and  comets,  so  as  to  show  them  as  they 
really  are,  standing  out  solidly  between  us  and  the  more  distant  background  of 
stars.  Efforts  are  now  being  made  to  get  similar  stereoscopic  pictures  of  the 
stars,  by  coupling  photographs  taken  at  intervals  of  many  years. 


FIG.   113.  —  FULL  MOON   SHOWING  RADIATING  STREAKS 


IGNEOUS   FORCES   ON  THE   MOON  245 

With  a  small  telescope  the  whole  surface  is  seen  to  consist 
of  solid  land  like  that  of  our  continents.  The  Moon  is  there- 
fore a  solid  sphere  (or  spheroid)  like  our  Earth,  but  without 
any  oceans  or  seas.  The  dark  patches  are  now  recognised  to 
be  tolerably  level  plains  of  a  darker  material  than  the  rest  of 
the  surface.  Some  of  these  plains  are  more  or  less  surrounded 
by  mountain  chains,  or  lesser  elevations. 

The  telescope  also  brings  into  view  a  large  number  of  smaller 
and  more  regular  circles  scattered  on  all  parts  of  the  visible 
surface.  Those,  however,  which  are  near  the  edge  of  the  disc 
are  distorted  into  ellipses  by  perspective.  The  largest  of  these 
rings  are  easily  seen  to  consist  of  a  circular  embankment  or 
rampart  surrounding  a  saucer-like  depression  in  the  centre  of 
which  there  is  often  an  irregular  conical  elevation. 

When  the  Sun  shines  obliquely  on  these  circular  ramparts, 
their  shadows  are  seen  extended  on  the  plains  beyond,  as  well 
as  on  the  floor  of  the  interior.  Where  they  are  numerous  the 
scene  reminds  one  of  a  plaster-of-paris  surface  that  has  been 
filled  with  bubble-holes  by  means  of  citrate  of  magnesia. 
When  such  a  surface  is  illuminated  from  one  side  by  a  power- 
ful electric  light,  it  makes  an  almost  perfect  representation  of 
some  parts  of  the  lunar  disc. 

Those  who  are  acquainted  with  volcanic  districts  on  our 
Earth  will  have  no  difficulty  in  recognising  these  hollow  cir- 
cular objects  as  volcanic  craters  with  or  without  central  lava 
cones. 

These  lunar  craters  are  extremely  numerous  and  vary  greatly 
in  size.  The  largest  are  considerably  over  one  hundred  miles 
across,  and  below  this  all  sizes  are  represented  down  to  invisi- 
bility, the  smallest  seen  being  less  than  a  mile  across. 

On  and  near  the  dark  plains  these  craters  are  tolerably 
plentiful,  but  toward  the  southern  part  of  the  Moon's  disc  they 
are  so  astonishingly  numerous  that  they  crowd  together  and 
overlap  one  another.  The  large  ones  are  often  filled  with,  and 
surrounded  by,  swarms  of  little  ones,  which  even  perch  on  the 
summits  and  slopes  of  their  ramparts. 


246     HOW  TO  KNOW  THE   STARRY   HEAVENS 

Several  of  the  larger  craters  form  centres  for  bright  star-like 
radiating  streaks  extending  along  the  surface  for  hundreds  of 
miles.  The  most  extensive  set  of  these  has  already  been  men- 
tioned as  giving  the  full  moon  the  appearance  of  a  peeled 
orange.  Its  longest  streak  can  be  traced  for  over  2,000  miles. 

With  a  powerful  telescope  long  narrow  cracks  are  also  visible 
on  the  Moon's  surface,  as  well  as  crust  foldings  and  all  kinds 
of  small  irregularities  which  need  not  be  here  described.  They 
are  beautifully  pictured  in  Nasmyth  and  Carpenter's  splendid 
but  expensive  work  entitled  "  The  Moon." 

IGNEOUS  FORCES 

Now  it  is  evident  that  most,  if  not  all,  of  the  lunar  features 
just  mentioned  are  of  exclusively  igneous  or  volcanic  origin.1 
There  is  no  single  feature  or  peculiarity  which  can  with  cer- 
tainty be  ascribed  to  either  air  or  water.  Oceans,  seas,  lakes, 
marshes,  rivers,  planes  of  denudation,  snow-patches,  ice-sheets, 
clouds,  mists,  and  such-like  aqueous  features  and  phenomena, 
are  conspicuous  by  their  total  absence. 

On  our  Earth  the  various  aqueous  agencies  have  for  millions 
of  years  waged  an  unceasing  conflict  with  the  igneous  agencies, 
and  have  now  almost  won  the  fight.  But  on  the  Moon  the 
igneous  forces  have  had  all  the  field  to  themselves,  and  have 
had  to  deal  with  a  force  of  gravitation  only  one  sixth  as  great 
as  that  on  our  Earth.  We  therefore  find  that  all  the  lunar 
features  are  such  as  would  be  produced  by  volcanic  forces 
working  under  peculiarly  favourable  conditions.  When  com- 
pared with  similar  volcanic  features  here  they  are  seen  to  be  on 
a  vastly  greater  scale,  differently  proportioned,  and  more  per- 
fectly preserved. 

Before  dealing  further  with  the  condition,  peculiarities, 
origin,  and  development  of  these  lunar  features,  it  may  be  well 
to  say  a  few  words  about  the  various  ways  in  which  volcanic 
forces  are  likely  to  act  under  widely  different  conditions.  We 

1  The  word  "  volcanic  "  is  here  used  in  its  widest  sense. 


IGNEOUS   FORCES   ON  THE   MOON  247 

shall  then  be  better  able  to  understand  the  unfamiliar  shapes, 
sizes,  and  other  peculiarities  of  the  lunar  relics  of  igneous 
action. 

HOW  A  MOLTEN  WORLD  SOLIDIFIES 

The  cooling  off  of  a  luminous  gaseous  sun  or  planet,  first  into 
a  liquid  and  then  into  a  solid  state,  is  an  exceedingly  protracted 
process.  An  appreciable  part  of  it  has  never  been  witnessed 
by  ephemeral  man.  This  being  the  case,  it  must  be  borne  in 
mind  that  the  following  account  is  almost  entirely  theoretical. 
No  one  can  prove  its  truth,  though  the  greater  part  of  it  is 
supported  by  many  forms  of  indirect  evidence.  The  most  that 
can  be  said  for  it  is  that  it  is  exceedingly  probable. 

After  a  gaseous  sun-like  world  has  cooled  off  sufficiently  to 
allow  its  most  combustible  elements  to  burn  themselves  into 
chemical  compounds,  it  still  continues  to  cool  off  by  radiation 
into  space.  So  in  time  the  bulk  of  its  material  leaves  the 
gaseous  state  and  forms  a  spherical  white-hot  molten  globe. 
This  is  extremely  dense  and  compressed  at  the  centre,  where 
the  heavier  elements  and  compounds  naturally  tend,  and  com- 
paratively thin  and  "  watery  "  near  the  surface,  where  the  pres- 
sure is  small.  When  the  molten  globe  is  of  any  considerable 
size  it  is  surrounded  by  an  atmosphere  composed  of  those  gases 
which  liquefy  only  at  a  low  temperature. 

If  the  world  under  consideration  is  only  a  subordinate  centre 
of  condensation,  it  still  continues  to  revolve  around  the  main 
centre,  as  in  its  gaseous  youth.  Its  rotation  not  only  continues, 
but  (if  not  counteracted  by  tidal  action)  increases  in  rapidity 
as  it  cools  and  shrinks.  And  any  tides  that  may  have  been 
previously  produced  in  it  by  large  neighbouring  bodies  will 
still  continue  to  affect  it. 

In  time  the  radiation  of  heat  into  space  will  chill,  and  even- 
tually freeze,  the  surface  of  the  molten  world.  The  time  occu- 
pied in  solidifying  will  depend  on  the  mass  of  the  cooling  world 
and  on  the  resulting  density  of  its  atmosphere. 

The  solid  outside  crust  will  gradually  get  thicker,  but,  if 


248     HOW   TO   KNOW   THE   STARRY   HEAVENS 

neighbouring  worlds  cause  any  considerable  tides  beneath,  it 
will  be  liable  to  be  fractured  as  soon  as  it  loses  some  of  its 
flexibility.  The  cooling  and  freezing  processes,  however,  go 
on  at  the  surface  without  interruption,  though  considerably 
hindered  by  the  more  or  less  violent  physical  and  chemical 
reactions  below. 

At  last  a  tolerably  solid  crust  is  formed,  covering  the  entire 
world  to  a  considerable  depth.  This  crust  is  all  the  time  en- 
croaching on  the  molten  nucleus  inside  it,  and  will  eventually 
replace  it  to  the  very  centre. 

But  before  that  can  take  place  there  will  be  a  terrible  struggle 
for  supremacy  between  the  cooling  and  crushing  shell,  and  the 
crushed  and  overheated  nucleus. 

For  the  laws  of  cooling  and  contraction  are  such  that  just 
before  a  liquid  turns  to  a  solid  it  swells  out  and  occupies  more 
room  than  it  did  before,  and  that  after  it  has  solidified  it  shrinks 
as  it  continues  to  cool  off.1 

Owing  to  these  peculiarities  the  solid  outside  crust  at  first 
contracts  faster  than  the  nucleus  inside  it.  The  strain  produced 
by  this  cause  (assisted  by  the  presence  of  confined  gases)  is 
tremendous  beyond  conception,  and  appalling  in  its  world-wide 
effects.  When  at  its  greatest  intensity  it  causes  the  world  to 
throb  and  quiver  from  centre  to  circumference.  The  crust  frac- 
tures and  gapes  from  pole  to  pole.  Near  the  centres  of  force 
the  surface-blocks  jump  like  the  lid  of  a  pan  full  of  boiling 
water.  The  molten  rock  and  its  confined  gases  (which  consist 
partly  of  the  vapour  of  water)  force  their  way  through  the 
cracks  and  gaping  fissures.  Where  their  escape  is  strongly  op- 
posed, they  issue  in  explosive  fountains  of  fiery  solids,  liquids, 
and  gases,  and  bombard  the  heavens  with  an  incessant  and 
ear-rending  cannonading  that  defies  description.  If  an  eye- 
witness could  view  the  stormy  scene  from  a  safe  distance,  he 
might  think  that  the  throes  of  Eagnarok  were  at  their  height. 

1  The  freezing  of  water  and  solidification  of  type-metal  sufficiently  illustrate 
the  former  peculiarity,  and  the  cooling  of  almost  any  solid  substance  exemplifies 
the  latter. 


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IGNEOUS  FORCES   ON  THE   MOON  249 

And  in  fact  he  would  not  be  far  wrong,  for  the  old  Norsemen 
probably  got  the  germ  of  that  grand  conception  from  the  world- 
rocking  convulsions  of  the  Icelandic  volcanoes. 

In  the  course  of  time  the  solidifying  process  gets  nearer  the 
centre  of  the  world.  The  igneous  forces  are  therefore  hampered 
by  the  increasing  weight  of  the  solid  crust  above.  The  molten 
nucleus  now  contracts  faster  than  the  solid  crust,  and  the  latter 
crumples  and  folds  as  it  settles  down  on  the  outside  of  it.  The 
strains  produced  by  both  contraction  and  expansion  vary  in  di- 
rection and  intensity,  and  the  surface  phenomena  vary  accord- 
ingly. The  igneous  manifestations  in  time  become  less  violent, 
and  the  conflict  gradually  dies  away  into  feeble  and  intermittent 
spasms,  which  at  last  entirely  cease. 

The  natural  features  resulting  from  the  above  conflict  vary 
according  to  the  intensity  of  gravitation,  the  density  of  the  at- 
mosphere, the  amount  of  water  above  and  below  the  surface, 
and  such  like  peculiarities,  and  these,  in  their  turn,  depend 
largely  on  the  size  of  the  cooling  sphere. 

TIME  OCCUPIED  IN  SOLIDIFYING 

On  a  little  world,  like  our  Moon,  for  example,  the  force  of 
gravitation  is  small.  Consequently  the  gases  and  vapours  appear 
to  leave  the  planet  as  they  are  liberated.  It  is  therefore  ex- 
posed to  the  intense  cold  of  outer  space,  and,  as  its  surface  is 
large  when  compared  with  its  mass,  it  cools  off  in  a  compara- 
tively short  time. 

On  a  somewhat  larger  world,  like  the  Earth  or  Venus,  gravi- 
tation is  more  powerful,  and  retains  all  the  gases  except  the 
very  lightest.  It  is  therefore  surrounded  by  a  dense,  foul,  and 
cloudy  atmosphere,  which  retards  and  lengthens  the  cooling 
process.  But,  as  soon  as  the  surface  temperature  is  low  enough 
the  aqueous  vapour  in  this  atmosphere  turns  to  liquid  drops 
and  rains  down  on  to  the  solid  surface.  At  first  this  surface  is 
too  hot  to  retain  the  water,  and  it  is  driven  off  again  in  the 
form  of  steam.  But  after  a  time  it  collects  into  rivers  of  hot 


250      HOW   TO   KNOW   THE    STARRY   HEAVENS 

water  and  flows  into  any  depressions  there  may  be  in  the  crust. 
There  it  forms  oceans,  seas,  and  lakes,  of  more  or  less  saline 
water.  This,  by  its  alternate  evaporation  and  condensation, 
produces  on  the  dry  land  all  the  well-known  phenomena  of 
aqueous  denudation. 

On  a  still  larger  world,  like  Saturn  or  Jupiter,  the  process  is 
still  more  hampered  and  prolonged  by  the  intensity  of  gravita- 
tion and  by  the  extent  and  density  of  the  cloud-packed  atmos- 
phere.1 

In  the  case  of  huge  bodies  like  our  Sun,  and  of  still  larger 
ones  like  Canopus  and  Rigel,  the  time  taken  to  cool  off  (first  to 
a  liquid,  and  then  to  a  solid  state)  is  so  enormous  that  it  is 
beyond  the  comprehension  of  ephemeral  beings.  Reason  may 
be  able  to  estimate  the  period  in  years,  but  even  the  imagination 
fails  to  grasp  and  realise  its  vastness. 

COMPARATIVE  EFFECTS  OF  IGNEOUS  FORCES 

When  this  conflict  was  at  its  height  on  our  Earth,  the  cen- 
tral attraction  of  gravitation  was  (as  now)  considerable  when 
compared  with  that  of  the  Moon,  and  there  was  a  dense  acrid 
atmosphere  all  around,  which  at  first  contained  all  the  water 
now  in  the  oceans.  These  features  would  act  as  a  check  on 
the  igneous  forces,  prolong  tbeir  action,  and  make  their  per- 
manent records  less  conspicuous.  Yet  geology  shows  that  some 
tremendous  results  were  produced  even  when  the  conflict  was 
dying  away. 

Leaving  the  more  ancient  and  important  effects  out  of  con- 
sideration, the  crater  of  Haleakala,  which  has  an  area  of  16 
square  miles,  is  an  awe-inspiring  evidence  of  past  volcanic 

1  The  low  density  of  the  large  outer  planets  shows  that  they  are  still  in  a 
gaseous  state  with  possibly  a  small  molten  nucleus.  Owing  to  their  rapid  rotation 
the  dense  clouds  by  which  they  are  surrounded  are  whirled  into  more  or  less  per- 
manent latitudinal  belts,  which  compose  the  only  visible  part  of  the  planet.  The 
belts  on  each  side  of  the  Equator  exhibit  more  frequent  changes  than  elsewhere. 
The  Sun  has  a  somewhat  similar  peculiarity,  as  the  sun-spots  and  eruptive  promi- 
nences are  mainly  confined  to  the  same  "  sub-tropical "  regions. 


IGNEOUS  FORCES   ON   THE   MOON  251 

power.  The  eruption  of  Krakatoa,  a  few  years  ago,  shook  the 
Earth  to  its  very  centre  and  from  pole  to  pole.  Its  upward 
cannonading  was  so  terrific  that  it  was  heard  3,000  miles  away, 
and  it  almost  shot  its  projectiles  into  outer  space.  The  Japan- 
ese earthquakes,  even  now,  are  active  enough  to  cause  the 
islands  to  be  facetiously  termed  "  The  Lid  of  Sheol."  Even  the 
sputterings  of  Mont  Pele'e  and  Vesuvius  are  sometimes  violent 
enough  to  inspire  our  respect.  Who,  then,  can  describe,  or 
even  imagine,  the  world-wrecking  violence  of  the  igneous  forces 
when  they  were  at  the  height  of  their  strength  and  activity, 
before  geology  began  ? 

On  a  much  larger  world  than  ours,  with  heavier  rocks  and  a 
still  denser  atmosphere,  the  igneous  forces  are  even  more 
cramped  than  here,  and  the  radiating  surface  is  smaller  in  com- 
parison with  the  mass.  The  process,  therefore,  continues  for  a 
much  longer  time,  and  the  resulting  evidences  are  compar- 
atively insignificant. 


FIG.  115.  —  SECTION  OF  EARTHLY  VOLCANOES 
Built  up  of  ashes  and  cinders,  with  interstratified  lava  streams. 

But  on  a  smaller  world,  like  our  Moon,  the  igneous  forces 
appear  to  be  practically  unfettered.  As  the  material  there  is 
only  one  sixth  as  heavy  as  it  would  be  on  our  globe,  it  can  be 
moved  with  only  one  sixth  the  effort. 

If  two  volcanoes,  one  on  the  Moon  and  the  other  on  the 
Earth,  were  each  to  eject  the  same  kind  of  material  with  the 
same  amount  of  force  and  under  the  same  atmospheric  condi- 
tions, the  lunar  projectiles  would  go  six  times  as  far  as  the 
earthly  ones.  But  as  there  is  no  resisting  atmosphere  on  the 
Moon,  the  lunar  volcano  will  scatter  its  debris  to  an  even 
greater  distance  than  the  difference  in  weight  would  indicate. 


HOW  TO  KNOW  THE   STARRY   HEAVENS 

The  result  is  that  a  violent,  explosive,  and  long-continued 
eruption  will  not  (as  here)  build  up  a  huge  mountain  of  debris 
with  a  small  crater  on  its  summit.  It  will  tear  out  an  enor- 
mous circular  funnel-shaped  hollow,  surrounded  by  a  massive 
rampart  just  at  the  edge  of  the  volcano's  field  of  force. 


FIG.  116.  —  SECTION  OF  LUNAR  VOLCANO  IN  FULL  ACTIVITY 

It  may  be  well  to  give  one  or  two  illustrations  of  these  dif- 
ferences in  size  and  proportion  between  the  earthly  and  the 
lunar  volcanoes. 

The  largest  volcanic  crater  on  our  globe  is  an  extinct  one  in 
the  Hawaiian  Islands,  that  goes  by  the  musical  name  of  Hal- 
eakala.  Its  size,  however,  has  been  increased  and  its  shape 
altered  by  an  explosion  like  that  which  blew  the  head  off  the 
original  Mount  Vesuvius.  The  crater  as  it  now  exists  is  7^ 
miles  across,  one  way,  and  2i  miles  the  other.  It  has  an  area 
of  about  16  square  miles,  and  the  depth  is  about  2,700  feet. 
There  are  16  conical  hills  scattered  about  its  interior,  from  500  to 
600  feet  in  height.  It  is  perched  on  the  summit  of  a  mountain 
20  miles  across  and  10,000  feet  above  the  sea-level. 

In  the  southern  part  of  the  Moon  is  a  crater  to  which  has 


FIG.    117.  —  CLAVIUS  AND   TYCHO 

The  former  is  the  large  walled  plain  near  the  bottom  of  the  picture.     The  latter  is  the 

centre  crater.     It  is  the  source  of  the  chief  lunar  radiating  streaks,  which  are 

only  visible  at  full  moon.     Photographed  at  Yerkes  Observatory. 


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IGNEOUS  FORCES   ON  THE   MOON  253 

been  given  the  name  of  Clavius.  It  is  one  of  the  largest  on  our 
side  of  the  Moon,  and  has  several  smaller  and  more  recent 
craters  scattered  about  its  interior.  It  is  more  than  140  miles 
across,  and  has  an  area  of  about  16,000  square  miles.  Its  ram- 
part in  some  places  reaches  a  height  of  17,000  feet  above  the 
inside  floor,  which  is  about  two  miles  below  the  level  of  the 
outside  plain. 

This  crater,  like  all  the  very  large  ones  on  our  side  of  the 
MOOD,  has  no  central  cone.  It  is  thought  by  some  that  these 
coneless  craters  are  not  the  result  of  explosive  volcanic  erup- 
tions, but  that  they  were  formed  as  seething  lakes  of  molten 
lava,  like  the  crater  of  Kilauea  in  the  Hawaiian  Islands. 

There  are  many,  however,  which  have  inside  cones,  indicating 
that  they  are  true  volcanic  craters.  It  may  be  as  well  to  give 
the  dimensions  of  one  of  the  largest  of  these. 

Theophilus,  one  of  the  most  perfect  craters  on  the  Moon,  is 
64  miles  across,  and  has  an  area  of  about  3,200  square  miles. 
Some  of  the  peaks  on  its  rampart  reach  an  elevation  of  18,000 
feet  above  the  floor  of  the  crater.  One  of  the  cone-shaped 
mountains  in  its  centre  is  6,000  feet  high,  yet  its  summit  is 
4,000  feet  below  the  level  of  the  outside  plain.  A  number  of 
similar  craters  are  scattered  around,  but  they  are  smaller  or  in 
a  less  perfect  state  of  preservation.  Besides  these,  there  are 
swarms  of  craters  so  small  that  only  a  very  powerful  telescope  will 
show  them.  But,  small  as  they  are,  they  are  equal  in  size  to  the 
craters  of  the  largest  earthly  volcanoes. 

VARIETIES  OF  IGNEOUS  ACTION 

The  solid  crust  which  first  forms  on  a  solidifying  world  is 
more  or  less  flexible  on  account  of  its  thinness  and  high  tem- 
perature. As  it  cools  off  and  loses  this  flexibility  it  is  cracked 
in  all  directions  by  the  tides  and  other  strains  to  which  it  is 
exposed.  The  cracks  are  filled  up  with  molten  rock,  which 
sometimes  reaches  the  surface  and  spreads  over  it  in  thin 
sheets.  As  this  liquid  matter  cools  and  consolidates,  it  cements 
the  crust  solidly  together  again.  The  process  goes  on  till  the 


254     HOW  TO   KNOW  THE   STARRY   HEAVENS 

whole  shell  is  traversed  in  every  direction  with  dykes  and 
veins  of  mineral  matter. 

When  the  crust  has  become  thicker,  and  the  interior  strains 
are  greater,  the  molten  matter  exudes  in  greater  volume  from 


FIG.  119.  —  SECTION  OF  MOUNTAIN  OF  EXUDATION 

the  cracks.  It  solidifies  quickly  on  the  cold  surface,  and  piles 
itself  up  in  great  heaps,  forming  ridges  and  sometimes  moun- 
tains of  exudation.  The  action  is  something  like  that  of  our 
mud- volcanoes,  only  the  material  is  plastic  through  heat  instead 
of  water. 

In  time  the  molten  nucleus  becomes  small,  in  proportion  to 
the  outside  shell.  It  then  contracts  faster  than  the  crust,  and 
leaves  the  latter  to  fold  and  shrink  upon  it  like  the  skin  of  a 


FIG.  120.  —  SECTION  OF  MOUNTAIN  OF  ELEVATION 

wizened  apple.  This  folding  or  crumpling  produces  what  are 
known  as  mountains  of  elevation ,  consisting  of  more  or  less 
parallel  ranges  of  folded  rock.1 

The  molten  matter,  instead  of  exuding  quietly  up  wide  cracks 
to  the  surface,  and  there  cooling  off  on  the  mountain  slopes,  is 

1  The  character,  distribution,  and  condition  of  these  folded  rocks  will  depend 
upon  whether  or  not  the  world  has  a  permanent  atmosphere  and  a  large  amount 
of  water  on  its  surface.  Also  on  whether  it  has  any  external  source  of  heat  and 
light,  to  keep  those  agencies  in  a  state  of  activity. 


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FIG.   118.  —  THEOPHILUS,  A  LUNAR  CRATER-WITH-CONK 

64  miles  across.     Yerkes  photograph. 


IGNEOUS  FORCES  ON  THE  MOON  255 

now  forcibly  ejected  from  a  series  of  blow-holes  along  the 
closed  or  choked  cracks.1  The  result  is  that  instead  of  piling 
up  mountains  of  exudation  it  forms  volcanoes  of  eruption. 

The  funnel-shaped  hollow  or  crater  formed  at  the  top  of  one 
of  these  blow-holes  is  surrounded  by  a  rampart  composed  of 
the  ejected  material.  While  the  explosive  force  is  increas- 
ing, the  hollow  gets  larger,  and  the  rampart  is  pushed  farther 
back.  The  size  of  the  crater  is  therefore  a  measure  of  the  energy 
of  the  volcano  at  the  height  of  its  power,  during  its  most 
violent  paroxysms. 

When  the  explosive  force  abates  somewhat,  small  craters  are 
formed  inside  the  large  one.  When  the  violent  eruptions  give 
way  to  a  quiet  yet  considerable  oozing  of  molten  rock,  it  fills 
up  the  crater  till  the  rampart  gives  way,  and  then  floods  the 
surrounding  country  with  more  or  less  liquid  lava.  When  there 
is  not  enough  lava  to  overflow  the  crater,  it  simply  solidifies 
inside,  forming  a  nearly  level  floor.  And  when  the  lava  flow  is 


FIG.  121.  —  SECTION  OF  LUNAR  CRATER  WITH  CONE 

too  small  or  viscid  to  spread  over  the  crater,  it  solidifies  into 
conical  peaks  above  the  central  vent. 

Sometimes  a  volcano  becomes  inactive,  not  because  the  energy 
is  exhausted,  but  because  it  is  intermittent  and  the  vent  has 
become  plugged  up  during  a  quiet  interval.  A  subsequent  dis- 
turbance, if  powerful  enough,  will  either  blow  out  the  obstruc- 
tion or  force  a  vent  in  some  other  part  of  the  line  of  weakness. 
In  the  latter  case  the  crust  will  be  fractured  afresh,  and  other 

1  The  violence  of  its  action  depends  largely  on  the  quantity  of  water  which 
gets  to  the  hot  interior,  and  on  the  volume  and  character  of  the  gases  produced 
by  chemical  action. 


256     HOW   TO   KNOW  THE   STARRY   HEAVENS 

craters  will  be  formed.  The  old  one  will  then  remain  dormant 
or  become  extinct. 

As  the  volcanic  forces  become  exhausted,  the  craters  formed 
are,  of  course,  smaller  than  the  old  ones.  And  so  at  last  the 
igneous  action  ceases  to  be  visible  on  the  surface  of  the  now 
solid  world. 

The  subsequent  history  of  the  world  in  question  will  depend 
upon  the  presence  or  absence  of  air  and  water  in  a  state  of 
activity.  With  them,  the  surface  structures  reared  by  igneous 
action  will  be  gradually  obliterated.  Without  them,  the  only 
changes  will  be  landslides  due  to  variations  of  temperature  act- 
ing on  the  piled-up  crater  ramparts  and  precipitous  mountain 
slopes.  With  this  exception  the  volcanic  structures  will  be 
perfectly  preserved  for  as  many  millions  of  years  as  the  petrified 
world  containing  them  continues  to  have  a  separate  existence. 


CHAPTER  XXII 

LUNAR  GEOLOGY  AND  GEOGRAPHY 

"  Speaking  by  our  own  lights,  from  our  own  experience  and  reasoning,  we  are 
disposed  to  conclude  that  in  all  visible  aspects  the  lunar  surface  is  unchangeable ; 
that  in  fact  it  arrived  at  its  terminal  condition  eons  of  ages  ago ;  and  that  in  the 
survey  of  its  wonderful  features,  even  in  the  smallest  details,  we  are  presented 
with  the  sight  of  objects  of  such  transcendent  antiquity  as  to  render  the  oldest 
geological  features  of  the  Earth  modern  by  comparison."  —  Nasmyth  and  Car- 
penter, "  The  Moon." 

BIRTH  OF  THE  MOON 

WHEN  the  Moon  still  formed  part  of  the  Earth,  the  whole 
mass  was  probably  in  a  molten  condition,  and  surrounded 
by  an  extremely  dense  atmosphere  which  contained  all  the 
water  now  in  the  oceans,  seas,  and  lakes.  The  atmospheric 
pressure  must  then  have  been  equal  to  about  5,000  pounds  to 
the  square  inch. 

Owing  to  the  rapid  and  increasing  rotation  the  molten  body 
was  very  much  spread  out  at  the  equator,  and  the  atmosphere 
was  deeper  and  denser  there  than  near  the  poles.  The  loss  of 
internal  heat  by  radiation  was  therefore  greatest  at  and  near  the 
poles.  This  probably  led  to  great  convectional  currents  in  the 
molten  globe.  The  same  cause  led  to  atmospheric  currents 
the  lowest  of  which  were  from  the  northeast  to  the  southwest 
in  the  northern  hemisphere,  and  vice  versa  in  the  southern. 

The  attraction  of  the  Sun,  acting  on  the  molten  rock  of  which 
the  rotating  spheroid  consisted,  presumably  raised  a  huge  and 
ever-increasing  tide  on  the  side  nearest  to  it.  As  the  rotation 
quickened,  this  solar  tidal  wave  of  molten  rock  grew  in  size 
until  it  was  finally  thrown  off  and  became  the  Moon,  as  de- 
scribed in  a  previous  chapter. 

17 


258     HOW   TO   KNOW   THE   STARRY   HEAVENS 

Now  the  material  which  went  to  form  the  Moon  was  not 
taken  from  the  heavy  central  metallic  rock  (8.2),  but  from  the 
lighter  envelope  of  molten  silicates,  with  a  density  of  a  little 
over  three  times  that  of  water  (3.2).  And  the  heavy  atmos- 
phere of  steam  and  other  gases  was  either  retained  or  drawn 
back  by  the  Earth  on  account  of  its  superior  size  and  attraction. 
The  lunar  silicates,  however,  were  probably  heavily  charged 
with  steam,  etc.,  owing  to  the  enormous  atmospheric  pressure  to 
which  they  had  previously  been  subjected. 

The  orbit  of  the  new-born  Moon  gradually  increased  in  size, 
from  the  tidal  action.  The  molten  silicates  composing  the 
Moon,  being  relieved  from  the  tremendous  atmospheric  pressure, 
gradually  expelled  the  steam  and  gases  with  which  they  were 
charged.  They  thus  formed  a  huge  globe  of  "boiling"  rock, 
which  cooled  off  on  every  side,  by  radiation  into  space,  until 
the  surface  matter  solidified. 

THE  GREAT  PLAINS 

The  escaping  gases,  assisted  by  the  earth-tides,  probably 
broke  up  the  first  crust,  and  the  fragments  floated  and  drifted 
on  the  still  molten  surface.  Large  open  lakes  of  boiling  rock 
may  thus  have  been  left  between  them.  These  molten  lakes 
would  be  something  like  that  in  the  crater  of  Kilauea,  but  of 
vastly  greater  dimensions.  They  were  sometimes  many  hun- 
dreds of  miles  across,  and  were  more  or  less  surrounded  by 
rocky  ramparts.  These  may  have  been  built  up  of  solid  blocks 
driven  to  the  sides  by  the  violent  ebullition  and  diverging 
surface  currents. 

In  the  course  of  time  these  hypothetical  molten  lakes  cooled 
off  and  solidified.  They  form  the  dark  level  plains  visible  from 
the  Earth  with  the  naked  eye.  Before  the  invention  of  the 
telescope  they  were  thought  to  be  seas ;  and  the  Latin  name 
for  a  sea  has  stuck  to  them  ever  since.1 

1  Even  Kepler  shared  in  this  erroneous  belief.  In  one  place  he  says  "Do 
maculas  esse  maria,  do  lucidas  esse  terras."  Galileo,  on  the  contrary,  appears  to 
have  discovered  that  there  are  no  visible  bodies  of  water  on  our  side  of  the  Moon, 


3    1 


LUNAR  GEOLOGY  AND  GEOGRAPHY   259 

Most  of  these  great  plains  have  either  a  circular  outline  or 
are  more  or  less  surrounded  by  semicircular  bays.  Mare 
Serenetatis  (the  Sea  of  Serenity)  is  an  example  of  the  former, 
and  Mare  Imbrium  (Sea  of  Showers)  of  the  latter.  This 
circular  feature  is  not  the  result  of  accident.  Whatever  differ- 
ences of  opinion  there  may  be  as  to  the  way  in  which  they 
originated,  it  is  certain  that  they  are  the  result  of  (igneous  or 
other)  forces  acting  from  local  centres. 

THE  WALLED  PLAINS 

Mare  Crisium,  one  of  the  smallest  and  most  regular  of  the 
great  plains,  connects  them  with  the  crater-like  watted  plains, 
of  which  the  best  examples  are  known  by  the  names  of  Ptolemy, 
G-rimaldiy  Clavius,  Schiller,  and  Schickard.  Most  of  these  are 
over  100  miles  in  diameter. 

In  spite  of  the  enormous  -surface  dimensions  of  these  walled 
plains,  and  of  the  fact  that  they  have  a  practically  level  floor 
with  no  central  cone,  their  appearance  is  so  crater-like  that  it  is 
impossible  to  draw  a  satisfactory  dividing-line  between  them 
and  the  smaller  volcanic  craters  which  were  subsequently 
formed.  Their  resemblance  is  so  marked  that  it  is  evidently 
due  to  relationship. 

RADIATING  STREAKS 

After  the  surface  had  cooled  off  and  consolidated,  the  cooling 
process  went  on  below,  adding  to  the  solid  crust  at  the  expense 
of  the  molten  nucleus.  The  varying  ratios  of  contraction,  etc., 
gave  rise  to  violent  strains  which  resulted  in  immense  cracks 
radiating  in  all  directions  from  the  local  centres  of  force.  The 
molten  lava  welled  up  through  the  cracks  and  spread  over  the 

for  his  friend  Milton  carefully  avoided  mentioning  lunar  oceans  and  seas.    He 
made  Raphael  watch  the  Earth  — 

"  as  when  by  night  the  glass 
Of  Galileo,  less  assured,  observes 
Imagined  lands  and  regions  in  the  Moon." 

—  "  Paradise  Lost,"  Bk.  V. 


260     HOW  TO   KNOW  THE   STARRY   HEAVENS 

surface  for  some  little  distance  in  thin  watery  sheets.  These 
of  course  soon  solidified,  and  are  now  visible  when  the  Moon  is 
at  its  full. 

MOUNTAINS  OF  EXUDATION 

At  a  later  time,  when  the  conditions  were  rather  different, 
the  solid  crust  appears  to  have  been  again  fractured  in  some 
parts  of  the  Moon,  and  immense  quantities  of  molten  lava 
welled  out  onto  the  surface.  There  it  cooled  off  and  solidified, 
like  spring  water  that  comes  to  the  surface  in  a  severe  frost. 
In  this  way  long  ridges  of  solidified  lava  were  formed,  with 
open  vents  all  along  the  summit.  Through  these  vents  the 
quiet  exudation  of  lava  continued  until  the  ridges  became 
mighty  ranges  of  mountains.  Such  are  the  Leibnitz  Mountains, 
in  the  extreme  south  of  the  Moon.  These  have  an  elevation 
of  31,000  feet  above  the  general  surface.  They  are  therefore 
higher  in  proportion  than  any  mountains  on  the  Earth. 

Another  important  range  is  known  as  the  Lunar  Apennines, 
a  little  to  the  north  of  the  centre  of  our, side  of  the  Moon. 
It  is  an  extremely  wild  and  precipitous  range  of  mountains 
about  480  miles  long  and  of  considerable  width.  It  contains 
about  3,000  peaks,  which  probably  represent  the  widest  parts 
of  the  elongated  cracks  through  which  the  liquid  silicates  rose 
to  the  surface.  Some  of  these  summits  are  18,000  feet  above 
the  level  of  the  plains  below. 

The  Caucasus  Range  and  the  Lunar  Alps  are  similar 
mountains  of  exudation.  The  latter  range  contains  700  peaks 
and  is  almost  cut  in  two  by  a  remarkable  straight  valley  with 
a  level  floor  and  precipitous  sides  11,000  feet  high.  It  is  80 
miles  long  and  about  5  miles  wide  at  the  bottom. 

With  perhaps  the  exception  of  the  first  mentioned,  all  these 
ranges  of  mountains  have  a  steep  precipitous  slope  toward  the 
Moon's  west,1  and  a  long  gradual  slope  on  the  side  toward 
which  they  are  carried  by  the  Moon's  rotation.  In  this  respect 
they  resemble  the  Andes  of  South  America. 

1  Toward  the  east  as  seen  from  the  Earth. 


liili 

:fi'l 


m  - 

•   • 


!OTs 

MmMM 


FIG.   124.  —  COPERNICUS 

By  Nasniyth  and  Carpenter.     (From  "  The  Moon,"  by  Nasmyth  and  Carpenter, 
published  by  John  Murray. ) 


LUNAR   GEOLOGY   AND  GEOGRAPHY        261 


ISOLATED  PEAKS 

A  few  isolated  peaks  were  also  formed,  perhaps  by  the  same 
quiet  exudation  of  rather  viscid  lava.  They  rise  rather  abruptly 
from  the  level  plain,  like  some  vast  cathedral.  There  is  one  in 
the  north  which  goes  by  the  name  of  Pico.  It  rises  almost 
precipitously  to  a  height  of  8,000  feet.1 

EXPLOSIVE  VOLCANIC  ERUPTIONS 

After  these  mountain  ranges  and  isolated  peaks  had  been 
more  or  less  quietly  formed  on  the  northern  plains  the  increas- 
ing shrinkage  of  the  nucleus  appears  to  have  closed  up  the 
wide  cracks  and  made  egress  to  the  surface  more  difficult. 
The  result  was  that  the  volcanic  forces  became  violent,  and  the 
molten  lava,  instead  of  quietly  oozing  up  all  along  the  cracks, 
was  forced  up  explosively  at  distant  centres.  Owing  to  the  feeble 
gravitation,  and  to  the  absence  of  atmospheric  pressure,  the 
resulting  volcanic  craters  were  so  tremendous  in  size  that  we 
have  nothing  on  Earth  anywhere  approaching  them. 

After  the  volcanic  forces  had  passed  their  maximum  the 
large  craters  were  choked  up,  and  smaller  ones  were  formed  on 
their  flanks,  to  be  in  time  superseded  by  still  smaller  ones. 
There  are  sometimes  strings  of  them,  evidently  formed  along 
the  same  crust-fractures. 

QUIET  VOLCANIC  EXUDATIONS 

The  explosive  volcanic  eruptions  were  sometimes  succeeded 
by  the  quiet  exudation  of  lava  through  the  same  volcanic  vents. 
Where  the  flow  was  very  abundant  the  crater  was  filled  till  the 
lava  broke  down  the  rampart  and  flooded  the  outside  plains. 
In  the  case  of  one  crater,  which  has  been  named  Wargentin, 
the  lava  rose  to  the  very  brim  of  the  crater  and  there  solidified, 
forming  a  round  table-mountain  53  miles  across. 

1  The  lunar  peaks  are  not  really  as  precipitous  as  one  would  think  from  the 
long  shadows  they  causet 


HOW  TO   KNOW   THE   STARRY   HEAVENS 

In  cases  where  the  flow  of  lava  was  less  abundant  the  craters 
were  partly  filled  up,  and  have  now  either  a  level  or  a  slightly 
convex  floor.1  Very  often  this  has  a  number  of  little  craters 
scattered  about  it,  and  has  also  a  cone,  or  series  of  cones,  over 
the  central  vent. 

As  no  two  of  the  lunar  craters  were  formed  under  exactly 
the  same  conditions,  it  is  not  surprising  to  find  that  each  one 
has  an  individuality  of  its  own.  Yet  if  we  except  the  great 
differences  in  size,  which  are  easily  explained,  it  will  be  found 
that  their  resemblances  are  much  more  marked  than  their 
differences.  There  is  not  the  shadow  of  a  doubt  as  to  their 
volcanic  origin,  and  geologists  can  learn  many  a  lesson  as  to 
the  history  of  the  Earth  by  studying  the  volcanoes  on  the 
Moon. 

OPEN  CRACKS 

After  all  volcanic  activity  had  died  away  on  the  Moon's  sur- 
face (and  probably  after  the  Moon  had  solidified  to  its  centre), 
large  numbers  of  surface  cracks  were  formed  by  the  continued 
contraction.  Many  of  them  are  of  such  a  tremendous  size  that 
they  are  visible  to  our  telescopes.  Some  can  be  traced  for 
more  than  a  hundred  miles,  being  from  one  to  two  miles  wide 
and  of  great  but  unknown  depth. 

These  open  cracks  in  some  cases  cut  clear  through  the 
previously  formed  craters.  Not  only  are  some  of  the  large 
and  early  craters  thus  intersected,  but  also  some  of  the  small 
and  more  recent  ones. 

With  the  formation  of  these  open  cracks  the  Moon's  activity 
apparently  ceased  and  it  became  what  it  is  to-day  —  and  what 
our  Earth  will  be  to-morrow  —  a  dead  world. 

TYPICAL  FEATURES 

The  study  of  lunar  geology  leads  naturally  to  lunar  geography, 
which  is  its  product.  The  principal  features  can  be  best  learned 
with  the  help  of  a  good  telescope,  assisted,  and  preceded,  by 

1  Due  to  imperfect  fluidity. 


t>'  *  iio 

x         *u  *)     .    i: 


* 

as 


FlO.     125.  —  SCHICKAUD     AND    WARGENTIN 

By  Nasmyth  and  Carpenter.     (From  "  The  Moon,"  by  Nasmyth  and  Carpenter, 
published  by  John  Murray.) 


LUNAR  GEOLOGY  AND  GEOGRAPHY   263 

the  study  of  lunar  charts  and  photographs.  It  may  he  well, 
however,  to  point  out  some  of  the  best  illustrations  of  the 
peculiarities  already  mentioned. 

The  G-reat  Plains  are  conspicuous  objects  on  account  of 
their  great  size  and  dark  hue.  Most  of  them  can  be  made  out 
with  an  opera  glass,  or  even  with  the  naked  eye.  Their  names 
will  be  found  on  the  lunar  charts  at  the  end  of  the  book. 

The  bright  streaks  which  have  been  mentioned  radiate  from 
the  craters  which  bear  the  celebrated  names  of  Tycho,  Coper- 
nicus, Aristarchus,  Kepler,  and  Proclus.  These  craters  de- 
veloped after  the  streaks  were  formed,  but  owe  their  positions 
to  the  weakness  and  open  state  of  the  crust  at  the  centre  of 
fracture. 

There  are  over  100  streaks  radiating  from  Tycho.  They 
spread  over  a  great  part  of  the  Moon's  surface.  One  of  them 
passes  under  the  distant  crater  Menelaus,  stretches  clear  across 
the  "  Sea  of  Serenity,"  and  is  finally  lost  to  sight  at  the  north- 
ern edge  of  the  Moon. 

The  streaks  radiating  from  Copernicus  are  next  in  importance. 
They  are  so  intricate  as  to  be  uncountable. 

The  Aristarchus  streaks  appear  to  have  been  formed  after 
those  of  Copernicus  but  before  those  of  Kepler. 

The  craters-with-cones  vary  in  size  from  the  giant  Petavius, 
78  miles  across,  to  the  little  companion  to  Hell,  1-f  miles  in 
diameter.  There  are  probably  many  smaller  ones  which  the 
telescope  fails  to  reveal. 

Copernicus,  56  miles  across,  is  one  of  the  grandest  objects  on 
our  side  of  the  Moon.  It  is  remarkable  not  only  for  its  radiat- 
ing streaks,  but  also  for  the  landslides  on  both  sides  of  its 
rampart,  for  its  radial  spurs  extending  for  100  miles,  for  the 
open  cracks  in  its  neighbourhood,  and  for  the  thousands  of  tiny 
craters  which  surround  it. 

Plato,  60  miles  across,  is  a  coneless  crater  easily  recognised, 
at  full  moon,  as  a  dark  oval  spot  near  the  northern  edge. 
Like  all  other  craters,  it  is  better  seen  when  the  sunshine  falls 
on  it  at  an  angle,  and  throws  the  black  shadows  of  its  sur- 


264     HOW  TO   KNOW  THE   STARRY   HEAVENS 

rounding  peaks  on  the  smooth  inside  floor  and  on  the  hill 
country  beyond.  It  has  a  considerable  landslide  on  the  inner 
side  of  its  rampart,  and  there  are  thirty  very  small  craters 
scattered  about  on  its  level  floor.  The  Lunar  Alps  are  close  by, 
cut  hi  two  by  the  curious  straight  valley  already  mentioned. 

Aristarchus,  28  miles  across,  is  as  easily  recognised  by  its 
brilliant  chalk-like  hue,  that  almost  dazzles  the  eye.  The  sur- 
face outside  appears  to  be  folded  into  parallel  ridges,  and  there 
is  a  curiously  contorted  crack,  not  far  away,  which  is  about  two 
miles  in  width. 

Ptolemy,  Alphons,  and  Arzacha,el,  near  the  centre  of  ou~ 
side  of  the  Moon,  form  one  end  of  a  more  or  less  continuoui 
chain  of  large  craters  extending  to  the  south.  Near  the  last 
named  is  a  huge  circle  with  a  scarcely  perceptible  rampart. 
Across  this  almost  smooth  circle  there  runs  what  looks  like  a 
straight  railroad  embankment,  about  60  miles  long  and  2,000 
feet  high.  From  its  artificial  appearance  it  is  commonly  known 
as  The  Railway. 

Every  part  of  the  Moon's  surface  is  full  of  objects  that  are 
interesting  when  seen  through  a  good  telescope  with  the  right 
illumination.  With  the  exception  of  the  bright  streaks,  all  the 
lunar  features  are  best  observed  when  they  are  near  the  chang- 
ing "  terminator,"  so  that  the  light  falls  on  them  at  an  acute 
angle. 


FIG.   126.  —  PTOLEMY,  ALPHONS,   AND  ARZACHEL 

By  Nasmyth  and  Carpenter.     (From  "  The  Moon,"  by  Nasmyth  and  Carpenter, 
published  by  John  Murray.) 


CHAPTER  XXIII 

INHABITED  WORLDS 

"  As  there  are  other  globes  like  our  Earth,  so  there  are  other  [family  groups] 
like  our  Solar  System.  There  are  self-luminous  suns  exceeding  in  number  all 
computation.  The  dimensions  of  this  Earth  pass  into  nothingness  in  comparison 
with  the  dimensions  of  the  Solar  System,  and  that  system,  in  its  turn,  is  only  an 
invisible  point  if  placed  in  relation  with  the  countless  hosts  of  other  systems. 
Our  Solar  System,  far  from  being  alone  in  the  Universe,  is  only  one  of  an  exten- 
sive brotherhood,  bound  by  common  laws  and  subject  to  like  influences.  Even  in 
the  very  verge  of  creation,  where  imagination  might  lay  the  beginning  of  the 
realms  of  Chaos,  we  see  unbounded  proofs  of  order,  a  regularity  in  the  arrange- 
ment of  inanimate  things,  suggesting  to  us  that  there  are  other  intellectual 
creatures  like  us,  the  tenants  of  those  islands  in  the  abysses  of  space." — Dr.  J.  W. 
Draper. 

ARE  OTHER  WORLDS  INHABITED? 

IN  ancient  times  our  Earth  was  supposed  to  be  the  only  world 
in  existence.  In  fact  the  Universe  was  thought  to  consist 
solely  of  our  Earth  and  its  appendages.  But  in  the  course  of 
time  it  was  discovered  that  the  Earth  is  only  one  member  of  a 
family  of  worlds,  and  that  there  are  other  families  of  worlds 
outside  of  our  own. 

These  two  discoveries  naturally  gave  rise  to  a  lively  discus- 
sion as  to  whether  those  worlds  are,  as  a  rule,  inhabited  by 
living  organisms. 

At  first,  pride  and  theological  prejudice  led  many  people  to 
deny  emphatically  the  existence  of  any  kind  of  life  on  other 
worlds.  Even  after  the  consideration  of  probabilities  had  led 
to  the  abandonment  of  this  position,  the  existence  of  rational 
life  was  strongly  contested. 

After  this  came  a  time  when  the  most  progressive  minds 
went  to  the  other  extreme.  They  claimed  that  life  probably 


266     HOW  TO   KNOW  THE  STARRY  HEAVENS 

exists  on  every  world  of  any  size,  at  a  certain  stage  in  its 
development. 

The  first  of  these  extreme  views  concerning  the  plurality  of 
worlds  has  long  been  abandoned  by  all  intelligent  and  well- 
informed  persons.  The  last  is  now  being  somewhat  modified 
by  astronomers.  They  consider  that,  while  it  is  absurd  to  sup- 
pose that  our  Earth  is  the  only  inhabited  world  in  the  Universe, 
the  opposite  extreme  is  not  supported  by  the  available  evidence 
as  to  the  conditions  existing  on  other  worlds. 

While  the  extreme  affirmative  side  was  in  the  ascendency, 
certain  imaginative  astronomers  (and  other  imaginative  persons 
who  were  not  astronomers)  stated  that  they  had  recognised 
indications  of  life  on  the  Moon.  A  little  later,  some  of  our 
very  best  observers  constructed  charts  of  Mars  showing  the 
dark  markings  on  it  connected  together  by  very  artificial-looking 
"  cobwebs."  Schiaparelli  gave  the  Italian  name  canali  (mean- 
ing "  channels  "  )  to  these  elusive  markings.  English-speaking 
people  mistook  the  Italian  canali  for  the.  English  word  "  canals," 
and  jumped  to  the  conclusion  that  Mars  is  inhabited  by  very 
enterprising  engineers  who  eke  out  a  small  water-supply  by 
means  of  large  irrigating  canals.  The  theory  was  afterward 
made  less  absurd  by  the  supposition  that  the  markings  were 
not  the  canals  themselves,  but  the  belt  of  vegetation  on  each 
side  of  the  main  ditch. 

Made  enthusiastic  by  this  supposed  identification,  one  or  two 
sensational  astronomers  went  a  step  farther,  and  asserted  that 
the  Martians  were  trying  to  attract  our  attention  by  displaying 
geometrical  signals  constructed  on  a  world-wide  scale. 

The  supposed  evidences  of  life  on  the  Moon  were  easily 
proved  to  be  wholly  imaginative.  In  one  rather  notorious  case, 
indeed,  they  turned  out  to  be  nothing  but  a  hoax,  gotten  up  to 
test  the  credulity  of  the  general  public. 

With  regard  to  the  "  canals  "  of  Mars,  the  belief  is  now  grow- 
ing stronger  among  astronomers  that  they  are  largely  subjective 
phenomena  —  that  in  fact  the  majority  of  them  are  optical 
delusions  caused  by  the  observers  over-straining  their  eyes  in 


INHABITED   WORLDS  267 

the  effort  to  make  the  best  use  of  their  telescopes.  The  rest  of 
them  are  probably  due  to  close  sequences  of  spots  too  faint  to 
be  separately  visible. 

The  truth  appears  to  be  that  our  optical  appliances,  powerful 
as  they  may  be,  are  yet  very  far  from  the  degree  of  perfection 
required  in  order  to  detect  evidences  of  life  in  other  worlds. 
The  probability  is  that  they  always  will  be  unequal  to  the  task. 
We  have  therefore  no  means  of  actually  proving  or  disproving 
the  theory  that  other  worlds  are  inhabited.  There  appears  to 
be  some  positive  evidence  that  the  Earth  is  inhabited,  and  that 
its  Moon  is  not.  But  apart  from  these  two  planets  we  have  no 
actual  positive  evidence  that  life  exists,  or  does  not  exist,  any- 
where in  the  Universe.  And  we  never  shall  have. 

This  being  the  case,  about  the  only  way  we  can  deal  with 
the  problem  is  to  try  to  find  out  the  conditions  necessary  for 
life  as  we  know  it,  and  to  see  whether  those  conditions  exist  on 
any  of  the  other  planets  hi  our  System.  We  shall  then  be  in  a 
better  position  for  speculating  on  the  probability,  or  otherwise, 
of  these  worlds  being  inhabited  by  living  organisms  allied  to 
those  on  our  Earth. 

When  that  question  is  disposed  of,  there  will  still  remain 
the  more  obscure  problem  whether  any  of  the  worlds  are 
tenanted  by  entirely  different  forms  of  life. 

ESSENTIALS  TO  LIFE  AS  WE  KNOW  IT 

Among  the  things  which  are  absolutely  essential  to  organic 
life  here  are  the  four  elements  known  as  carbon,  hydrogen, 
nitrogen,  and  oxygen. 

Of  these,  oxygen,  in  a  free  gaseous  state,  is  essential  to  all 
forms  of  animal  life. 

Carbon  and  oxygen,  in  combination,  form  carbon-dioxide 
(C02),  which,  as  a  gas,  is  equally  essential  to  all  forms  of 
vegetable  life. 

Hydrogen  and  oxygen  combine  to  form  water  (H20),  which 
both  plants  and  animals  require  in  a  liquid  state. 


268     HOW   TO   KNOW  THE   STARRY  HEAVENS 

Carbon  and  hydrogen  form  the  basis  of  numerous  compounds 
which,  by  their  formation  and  oxidation,  alternately  accumulate 
energy  and  expend  it  in  motion,  etc.  These  two  processes  are 
together  known  by  the  name  of  energy-traffic. 

Nitrogen  is  necessary,  in  a  free  gaseous  state,  to  keep  in 
check  the  all-devouring  energy  of  free  oxygen.  It  is  also  neces- 
sary, in  combination  with  the  hydrocarbons,  to  regulate  and 
control  their  energy-traffic.  It  is  specially  adapted  for  this 
office  on  account  of  its  sensitiveness  to  changes  of  energy  and 
the  resulting  instability  of  its  compounds. 

All  the  life  on  our  globe  is  based  on  protoplasm,  which  con- 
sists of  these  four  elements.  A  number  of  other  elements 
occur  in  most  forms  of  life,  but  do  not  seem  to  be  so  essential 
to  it. 

An  intermittent  supply  of  radiant  energy  also  appears  to  be 
necessary  to  life  as  we  know  it.  The  plants  derive  their 
energy,  either  directly  or  indirectly,  from  the  Sun,  and  the 
animals  get  it  at  second-hand,  from  the  plants. 

The  energy-traffic  mentioned  above  can  only  be  carried  on 
within  a  certain  range  of  temperature.  Life  based  on  proto- 
plasm cannot  exist  where  the  temperature  is  permanently 
above  150°  K,  or  below  32°  F.  High  temperatures  break  up 
the  protoplasm  into  less  complex  compounds.  Low  temper- 
atures check  and  eventually  put  a  stop  to  its  activity. 

A  certain  intensity  of  the  force  of  gravitation  also  seems  to 
be  essential  to  life  as  we  know  it.  For  worlds  which  are  very 
much  smaller  than  ours  do  not  appear  to  be  able  to  retain  the 
vapours  and  gases  necessary  to  living  organisms,  and  on  worlds 
that  are  very  much  larger,  the  same  organisms  would  be 
crushed  to  death  by  their  own  weight. 

DO  LIFE-ESSENTIALS  EXIST  ON  OTHER  WORLDS? 

When  we  examine  the  other  planets  in  our  System,  we  find 
that  some  of  these  essentials  are  lacking  in  every  one,  and 
always  will  be  lacking. 


0°  Fastigium  Ceryor 


30°  Margaritifer-Sinus 


60°  Ganges 


90°  Solis  Lasus 


120°  Nodus  Gordii 


150°  Mare  Sirenum 


FIG.  127.  —  TWELVE  VIEWS  OF  MARS 

By  Lowell.     (From  Todd's  "  Stars  and  Telescopes,"  published  by 
Messrs.  Little,  Brown,  &  Co.) 


180°  Atlantis 


210°  Trivium  Charontis 


300°  Syrtis  Major  330°  Phisoner  Euphrates 

FIG.  127. —TWELVE  VIEWS  OF  MARS 

By  Lowell.     (From  Todd's  "Stars  and  Telescopes," published  by 
Messrs.  Little,  Brown,  &  Co.) 


•f\BR  ATTp 
Of   THE 

UNIVERSITY 

or 


INHABITED  WORLDS  269 

Our  nearest  neighbour,  the  Moon,  shows  no  signs  of  having 
either  air  or  water.  This  is  probably  due  to  the  fact  that  it  is 
unable  to  retain  them,  except  in  a  solid  state.  Owing  to  their 
absence,  and  to  the  slowness  of  the  planet's  rotation,  the  range 
of  temperature  is  too  great  for  any  form  of  life  based  on  the 
carbon  compounds. 

Mercury  and  Mars  probably  have  a  very  scanty  supply  of 
both  air  and  water.  There  are  no  perceptible  oceans  or  seas  on 
Mars,  and  Dr.  Campbell,  of  Lick  Observatory,  has  shown  that 
the  atmospheric  pressure  on  its  surface  is  very  small.  It  ap- 
pears, in  fact,  to  be  less  than  that  at  the  summit  of  Mount 
Everest,  the  highest  mountain  on  our  globe.  This  being  the 
case,  it  is  possible  that  the  planet  is  unable  to  retain  the  vapour 
of  water,  and  that  its  "  snowy  poles  "  are  nothing  but  frozen 
carbon-dioxide  (C02),  an  inch  or  two  in  thickness. 

Venus  probably  has  a  good  supply  of  both  air  and  water. 
But  as  it  appears  to  keep  one  side  always  turned  to  the  Sun,  it 
has  not  the  intermittent  supply  of  radiant  energy  which  is  nec- 
essary for  life  on  our  globe.  The  Sunward  side  must  be  too 
hot  for  protoplasm  to  exist,  and  the  dark  side  must  be  cold 
enough  to  freeze  both  water  and  air.  Mercury  also  appears  to 
possess  the  same  unfavourable  coincidence  between  its  rotation 
and  revolution. 

The  Asteroids  receive  much  less  light  and  heat  than  the 
Earth.  They  have  probably  neither  air  nor  water,  as  their 
gravitation  is  too  small  to  prevent  these  from  escaping  into 
space,  as  free  hydrogen  does  from  our  Earth. 

Jupiter  may  be  said  to  be  all  atmosphere,  as  it  appears  to  be 
still  in  a  hot  gaseous  state,  surrounded  by  thick  cloudy  conden- 
sations. The  intensity  of  the  Sun's  light  and  heat  is  27  times 
less  there  than  on  our  Earth.  Its  five  moons  must  therefore 
appear  as  mere  wandering  stars. 

Saturn  is  not  as  dense  even  as  Jupiter.  And  it  is  still  less 
favourably  situated  as  regards  solar  radiation,  in  spite  of  its 
enormous  satellite-rings  and  numerous  moons.  For  the  intensity 
of  solar  light  and  heat  is  90  times  less  than  here. 


270    HOW  TO   KNOW  THE   STARRY   HEAVENS 

Uranus  and  Neptune  are  little  more,  at  present,  than  huge 
masses  of  rotating  gas,  slowly  revolving  in  the  gloomy  outskirts 
of  the  Solar  System.  The  intensity  of  Neptune's  sunshine 
is  900  times  less  than  that  which  we  enjoy.  Its  lot  would 
therefore  be  a  cold  one  if  it  were  not  for  its  store  of  internal 
heat,  continually  replenished  by  the  gradual  contraction  of  the 
planet. 

This  concludes  our  survey  of  the  Solar  System  in  search  of 
the  essentials  of  life.  If  we  wait  until  the  outer  planets  have 
reached  the  present  stage  of  the  Earth,  we  shall  still  find  the 
conditions  utterly  unfavourable  to  life  as  we  know  it.  For  the 
force  of  gravitation  will  still  be  tremendously  greater  than 
here,  and  the  radiant  energy  from  the  distant  and  waning  Sun 
will  then  be  insufficient  to  keep  the  otherwise  dense  atmos- 
pheres from  settling  down  over  their  entire  surfaces,  in  a  solid 
mass  of  never-melting  "  snow." 

We  have  then  good  reason  to  think  that  in  our  own  System 
the  Earth  is  the  only  planet  on  which  the  conditions  are 
favourable  for  life  as  we  know  it.  That  is  to  say,  if  life  must 
necessarily  be  based  on  protoplasmic  combinations  of  oxygen, 
hydrogen,  nitrogen,  and  carbon,  the  other  planets  in  our  System 
appear  to  be  uninhabitable. 

Among  the  many  millions  of  solar  systems  which  surround 
our  own,  it  may  be  regarded  as  certain  that  there  is  no  single 
world  where  the  conditions  sue  identical  with  those  on  Earth. 
But  it  is  possible  that  there  may  be  quite  a  number  of  worlds 
where  the  conditions  are  somewhat  similar  to  those  found 
here. 

In  such  worlds  we  should  probably  find  living  organisms 
based  on  the  same  four  elements,  but  developing  along  some- 
what different  lines.  For  earthly  forms  of  life  have  grown  up 
under  certain  conditions,  and  cannot  exist  where  those  condi- 
tions do  not  prevail.  On  other  planets  life  would  start  under 
different  conditions  and  develop  accordingly.  In  other  words, 
the  various  forms  of  life  on  our  globe  are  the  result  of  general 
laws  operating  under  special  conditions,  and  the  same  general 


INHABITED  WORLDS 

laws  would,  under  different  conditions,  result  in  different  forms 
of  life. 

But  although  the  number  of  such  worlds  in  the  Universe  may 
be  numerically  large,  it  must  be  relatively  small.  We  know 
that  the  Universe  contains  hundreds  of  millions  of  luminous 
suns,  and  we  have  reason  to  believe  that  they  are  many  times 
outnumbered  by  dark  planetary  worlds.  Let  us  deduct  from 
these  latter  the  comparatively  small  number  on  which  the  con- 
ditions are  presumably  somewhat  similar  to  those  on  our  Earth. 
The  question  now  remains  whether  the  rest  are  entirely  with- 
out life  of  any  kind,  or  whether  they  are  tenanted  by  forms  of 
life  which  are  based  on  other  elements  and  require  altogether 
different  conditions. 

For  my  part,  I  must  say  that  the  latter  seems  the  most  rea- 
sonable supposition,  considering  the  resourcefulness  of  Nature 
in  our  part  of  the  Universe.  Yet  we  have  reason  to  believe 
that  even  our  World  is  entirely  uninhabited,  except  on  the  thin 
surface  layer.  And  it  is  only  habitable  there  for  a  compara- 
tively snort  time.  Nature  may  be  as  wasteful  of  worlds  as  it 
is  of  sunshine. 

The  fact  is  that  we  none  of  us  know  anything  at  all  about 
this  last  question.  And  we  never  shall  know. 

WHERE  IS  THE  EARTH? 

"  Beelzebub.     '  There  is  a  place 
(If  ancient  and  prophetic  fame  in  heaven 
Err  not),  another  world,  the  happy  seat 
Of  some  new  race,  called  Man. 

Thither  let  us  bend  all  our  thoughts,  to  learn 
What  creatures  there  inhabit,  of  what  mould 
Or  substance,  how  endued,  and  what  their  power.' 

Satan.    '  Whom  shall  we  send 

In  search  of  this  new  world?     Whom  shall  we  find 

Sufficient?     Who  shall  tempt  with  wandering  feet 

The  dark,  unbottomed,  infinite  abyss, 

And  through  the  palpable  obscure  find  out 

His  uncouth  way,  or  spread  his  aery  flight 


HOW  TO   KNOW   THE   STARRY   HEAVENS 

Upborne  with  indefatigable  wings, 
Over  the  vast  abrupt,  ere  he  arrive 
The  happy  isle? 

I  abroad 

Through  all  the  coasts  of  dark  destruction  seek 
Deliverance  for  us  all :  this  enterprise 
None  shall  partake  with  me.'"—  Milton,  "Paradise  Lost,"  Bk.  II. 

So  far  the  entire  subject  of  inhabited  worlds  has  been  treated 
from  an  earthly  standpoint.  It  may  be  an  advantage  to  con- 
sider it  briefly  from  the  outside. 

Let  us  suppose  for  a  moment  that  our  Earth  is  the  only 
world  in  the  Universe  which  is  inhabited  by  living  organisms. 
And  let  us  suppose  that  from  some  distant  spirit-land  a  flying 
messenger  is  sent  to  hunt  up  our  World,  without  any  special 
instructions  for  finding  it.  Let  us  also  suppose  that  he  can 
travel  through  space  at  a  speed  equal  to  that  of  light.  He  may 
travel  from  star  to  star,  for  hundreds,  thousands,  and  millions 
of  years,  without  being  able  to  find  it.  "  If  in  the  course  of  time 
he  comes  to  our  System,  he  may  see  the  larger  and  more  showy 
planets,  and  yet  fail  to  notice  our  little  World. 

If  it  were  to  be  pointed  out  to  him  as  the  specially  favoured 
world,  he  would  hardly  be  able  to  convince  himself  of  the  fact 
without  a  close  examination.  For  the  only  unusual  feature 
about  it,  as  seen  from  a  distance,  would  be  that  it  was  attended 
by  a  satellite  which  was  large  in  proportion  to  its  primary. 
Apart  from  this  peculiarity,  which  may  be  common  in  other 
systems,  the  Earth  would  seem  to  be  merely  an  insignificant 
attendant  on  a  star  which  was  itself  almost  indistinguishable 
from  millions  of  other  stars. 

That  our  Earth  is  the  only  inhabited  world  is  therefore  an 
extremely  unlikely  supposition  to  be  entertained  by  anyone 
who  is  at  all  conversant  with  the  dimensions,  construction,  and 
duration  of  the  Universe.  It  appears  reasonable  only  to  one 
who  is  almost  entirely  ignorant  of  all  except  his  immediate 
surroundings. 

Yet  there  are  people  on  our  Earth  (even  in  so-called  enlight- 


INHABITED  WORLDS  273 

ened  and  civilised  countries)  who  not  only  think  that  it  was 
created  expressly  for  the  use  of  man,  but  also  believe  that  the 
entire  Universe  —  visible  and  invisible  —  was  made  for  his 
service  and  edification  !  There  are  persons  who  actually  believe 
that  the  Sun  was  made  to  rule  an  earthly  day,  the  Moon  to 
adorn  an  earthly  night,  the  planets  to  serve  as  oracles  of  earthly 
fortunes,  and  the  stars  to  relieve  the  monotony  of  an  earthly 
sky !  The  sciences  of  astronomy  and  geology  show  that  — 
natural  as  these  ideas  may  seem  to  unenlightened  people  — 
they  are  as  baseless  as  the  airy  fabrics  of  a  midnight  dream. 

SUMMARY 

Dr.  F.  J.  Allen,  in  a  recent  article  in  the  "  Popular  Science 
Monthly," l  has  ably  summed  up  the  argument  as  to  "  Life  in 
Other  Worlds."  He  says  : 

"  1.  If  life  is  essentially  a  function  of  the  elements  nitrogen,  oxy- 
gen, carbon,  and  hydrogen,  acting  together,  then  it  can  probably 
occur  only  on  exceptional  worlds,  with  conditions  closely  resembling 
those  of  our  own  Earth.  Such  conditions  are  not  present  in  any 
other  world  in  our  own  Solar  System,  nor  can  they  be  expected  to 
occur  frequently  in  members  of  other  systems. 

"  2.  On  the  other  hand,  if  different  conditions  can  awaken  a  capac- 
ity for  exalted  energy-traffic  among  other  elements  than  those  just 
named,  then  the  Universe  seems  to  provide  immense  possibilities  of 
life,  whose  variety  and  magnificence  may  far  exceed  anything  that  we 
can  imagine." 

OTHER-WORLD  SPECULATIONS 

I  think  that  it  has  now  been  satisfactorily  shown  that  we 
have  no  direct  knowledge  whatsoever  concerning  inhabited 
worlds  outside  of  our  own,  and  that  we  are  never  likely  to 
obtain  such  knowledge. 

But  on  the  other  hand  it  has  also  been  shown  that  it  would 
be  preposterous  to  suppose  our  Earth  to  be  the  only  inhabited 

1  November,  1903. 

18 


274     HOW   TO   KNOW   THE   STARRY   HEAVENS 

world  in  the  Universe.  As  a  matter  of  probability,  we  may 
safely  take  it  for  granted  that  there  are,  at  the  present  time, 
myriads  of  inhabited  worlds,  a  certain  percentage  of  which 
have  developed  reasoning  beings.  We  may  also  take  it  for 
granted  that  there  have  been  myriads  of  such  worlds  in  the 
limitless  past,  and  will  be  in  the  limitless  future. 

This  being  so,  the  question  naturally  arises,  is  the  life  on 
those  worlds  likely  to  develop  along  the  same  lines  as  here, 
or  has  each  world  its  own  special  line  of  development,  not  con- 
ceivable by  the  inhabitants  of  any  other  planet  ?  In  other 
words,  is  it  probable  that  a  visitor  to  one  of  those  worlds  would 
there  find  two  ascending  series  of  organisms,  the  one  culmi- 
nating in  flowering  shrubs  and  trees,  and  the  other  in  back- 
boned quadrupeds,  headed  —  and  more  or  less  controlled  —  by 
a  reasoning  biped  like  man  ? 

At  first  sight  it  does  not  seem  as  though  any  time  could  be 
profitably  spent  in  considering  the  question.  It  looks,  indeed, 
as  though  it  would  be  only  another  case  of  arguing  about  the 
politics  of  the  people  who  live  —  or  do  not  live  —  on  the  other 
side  of  the  Moon. 

Yet  a  close  study  of  the  development  of  life  on  our  Earth 
appears  to  afford  considerable  indirect  and  circumstantial 
evidence  that  the  course  of  organic  evolution  may  be  very 
similar,  even  where  the  physical  and  chemical  conditions  are 
widely  different.  This  similarity  is  rendered  probable  by  the 
fact  that  organic  peculiarities,  both  internal  and  external,  have 
not  been  developed  so  much  by  the  inorganic  as  by  the  organic 
surroundings.  The  evolution  is  mainly  the  result  of  a  struggle 
for  existence,  which  must  go  on  wherever  there  is  life.  There- 
fore if  the  physical  environment  be  a  tolerably  permanent  one, 
favouring  the  processes  of  organic  chemistry,  the  resulting  life- 
forms  may  be  similar,  whatever  may  be  the  temperature  or  the 
active  chemical  elements. 

On  our  Earth,  Nature  has  long  been  unconsciously  experi- 
menting with  innumerable  kinds  of  organic  life.  Without 
either  mercy  or  spite  she  has  pitted  one  form  against  the  other 


Fi<;.    128.  —  Disc  OF  THE   SUN,    AUGUST  12,  1903 

Showing  brilliant  calcium  flocculi.     Middle  (H)  level.     Taken  with  Hale'a  spectroheliograph, 
at  Yerkes  Observatory. 


\TBRA;?> 

0«-   r-<E      ' 

UNIVERSITY 

or 
£*J 


INHABITED   WORLDS  275 

under  a  great  variety  of  local  conditions,  allowing  the  fittest  to 
survive  and  the  rest  to  perish.  The  "  experiments  "  have  been 
ceaselessly  carried  on  for  many  millions  of  years,  with  a  super- 
abundance of  material,  and  with  a  physical  and  chemical 
environment  that  has  been  almost  unchanging. 

This  being  the  case,  the  course  of  development  and  final 
result  may  have  been  inevitable  rather  than  the  effects  of 
chance.  Earthly  forms  of  life  may  be  practically  duplicated 
on  a  million  worlds  —  that  were,  and  are,  and  are  to  be. 

There  is  not  room  in  this  chapter  to  go  into  details  on  the 
question,  and  it  would  be  out  of  place  in  any  other  part  of  this 
volume.  But  a  few  hints  may  be  given  as  to  the  way  in  which 
organic  life  is  likely  to  develop  on  a  world  where  the  condi- 
tions prove  favourable. 

Whatever  the  temperature  and  active  elements  may  be,  it 
seems  certain  that  organic  life  must  be  based  on  the  cell  or  its 
equivalent,  and  that  the  first  cells  must  originate  from  and  in 
non-living  compounds  of  those  active  elements. 

A.  The  first  living  forms  must  be  single-celled  PROTISTA, 
having  their  home  in  water  or  some  similar  fluid. 

The  energy  necessary  to  carry  on  the  processes  of  life  must 
come  from  a  chemical  change  similar  to  the  oxidation  of  tissues. 
The  wasting  tissues  must  be  renewed  by  the  absorption  of 
soluble  inorganic  compounds. 

Each  single-celled  organism  must  go  through  a  life-cycle 
ending  either  in  death  or  in  division.  This  must  lead  to  re- 
production by  fission,  or  by  some  similar  process  involving  a 
less  expenditure  of  energy  than  the  original  spontaneous 
generation.1 

The  multiplying  individuals  must  adjust  themselves  to  local 
conditions,  and  thus  give  rise  to  varieties,  species,  etc.  When 
they  have  multiplied  till  food  becomes  scarce,  some  will  be 
compelled  to  feed  on  their  neighbours.  Only  those  will  survive 
which  can  obtain  food  and  protect  themselves  from  being 
eaten.  The  result  of  this  "  cannibalism  "  is  a  division  into 

1  And  capable  of  being  carried  on  under  less  favourable  conditions. 


276     HOW  TO   KNOW  THE   STARRY   HEAVENS 

plants,  which  live  on  soluble  inorganic  food,  and  animals, 
which  live,  either  directly  or  indirectly,  on  the  plants. 

When  single-celled  possibilities  have  all  been  long  and 
fully  tried,  the  only  possible  advance  will  be  found  in  combina- 
tion. Some  of  the  dividing  organisms  will  remain  in  contact 
and  partnership,  giving  rise  to  many-celled  plants  and  animals. 
These  compound  individuals  will  —  by  co-operation  and  division 
of  labour  —  find  greater  safety  and  economy,  resulting  in  a 
better  living  than  their  simple  competing  neighbours.  They  will 
therefore  get  the  upper  hand,  and  continue  to  advance  in  size 
and  complexity  of  organisation. 

Nature  may  be  said  to  have  issued  two  irrevocable  decrees 
with  regard  to  organic  evolution.  One  of  these  is  that  "  where 
there  is  no  necessity  there  shall  be  no  development."  The 
other  is  that  "  where  there  is  necessity  there  shall  be  either 
development  or  death." 

The  plants  —  living  by  the  absorption  of  soluble  inorganic 
compounds,  which  are  all  around  them  —  require  no  special- 
sense  organs,  or  apparatus  for  thinking,  moving,  eating,  digest- 
ing, etc.,  and  they  do  not  develop  them.  But  they  are  usually 
compelled  to  anchor  themselves  to  the  sea-bottom  where  the 
surroundings  are  favourable,  to  spread  themselves  out  so  as  to 
obtain  a  large  absorbing  surface,  to  protect  themselves  from 
their  animal  foes,  and  to  modify  their  methods  of  growth  and 
reproduction  wherever  the  local  conditions  require  it. 

The  animals  —  being  compelled  to  find  suitable  organic  food 
or  starve  —  will  develop  means  of  locomotion  and  all  the 
physical  and  mental  peculiarities  necessar}^  for  obtaining  and 
utilising  the  available  supply.  Like  the  plants,  they  will  find 
it  necessary  to  protect  themselves  from  their  animal  foes,  and 
to  modify  their  methods  of  growth  and  reproduction  wherever 
the  local  conditions  require  it. 

The  animals  will  therefore  rise  to  a  higher  plane  than  the 
vegetables,  and  the  free-moving  and  offensive  forms  will  develop 
more,  in  every  direction,  than  the  fixed,  sluggish,  and  defensive 
forms, 


INHABITED   WORLDS  277 

So  far  as  shape  is  concerned,  there  appear  to  be  only  two 
general  organic  types  possible,  either  here  or  in  any  other 
world.  These  are  the  radial  and  the  elongated  bilateral. 

Among  the  animals,  the  radial  type  has  given  rise,  on  our 
planet,  to  two  branches. 

B.  One  of  these  is  termed  the  CCELENTERATA.  It  contains 
such  forms  as  the  jelly-fishes,  sea  anemonies,  and  corals.  These 
have  only  one  opening  to  the  stomach. 

0.  The  other  branch  is  the  ECHINODERMATA.  It  includes 
the  crinoids,  starfishes,  and  sea-urchins.  They  have  the  stomach 
open  at  both  ends. 

All  the  forms  in  these  two  branches  are  either  fixed  or 
sluggish.  Other-world  forms  of  this  radial  type  would  probably 
be  the  same. 

The  elongated  bilateral  type,  although  very  unpromising  in 
its  earlier  stages,  has  much  greater  possibilities.  In  fact  all 
our  higher  forms  of  animal  life  belong  to  this  type. 

D.  The  simplest  form  is  that  of  a  marine  worm,  which  is 
little  more  than  a  long  sack  with  a  hole  at  each  end  of  it.     At 
this  stage  it  is  naturally  a  very  sluggish  animal. 

E.  When  the  worm-like  animal  develops  tentacles  or  arms 
around  its  mouth,  it  becomes  a  MOLLUSK,  represented  on  Earth 
by  the  clams,  snails,  and  cuttle-fish.     Some  of  these  forms  are 
rather  more  active  than  those  already  mentioned,  but  the  limit 
of  advancement  is  soon  reached. 

F.  When  the  worm-like  form  develops  a  segmented  structure, 
with  a  tough  skin,  and  numerous  jointed  legs  arranged  along 
its  two  sides,  it  becomes  the  more  active  ARTHROPODA,  repre- 
sented in  our  seas  by  the  lobster  family  and  some  insects. 

Gr.  When  the  same  worm-like  form  retains  its  simpler 
structure  but  develops  an  internal  skeleton,  with  a  tail  and  two 
pairs  of  lateral  limbs,  it  becomes  a  VERTEBRATE  fish-like  ani- 
mal. This  is  capable  of  great  activity,  and  appears  to  be  the 
highest  form  of  life  which  can  be  developed  under  oceanic 
conditions.1 

1  For  many  millions  of  years  this  fish-like  vertebrate  has  made  very  little 
progress  with  us,  in  spite  of  the  relative  immensity  of  our  oceans  and  the  great 


278     HOW  TO   KNOW  THE   STARRY  HEAVENS 

From  this  it  appears  probable  that  on  those  worlds  which 
are  nearly  or  entirely  covered  by  water  or  some  similar  liquid, 
the  highest  form  of  life  will  be  a  rather  stupid  fish-like  animal 
with  an  internal  skeleton,  simple  organisation,  a  tail,  and  two 
pairs  of  lateral  fins. 

On  those  planets  which  have  large  masses  of  land  rising  oat 
of  the  ocean,  some  forms  of  life  will  be  gradually  crowded  out 
of  the  water  and  modified  so  that  they  can  live  without  being 
constantly  submerged.  In  the  course  of  time  both  plants  and 
animals  will  spread  over  the  continents  and  islands,  relying  on 
an  occasional  rainstorm  to  keep  them  from  drying  up. 

On  our  Earth,  only  the  elongated  worm-like  type  sent  repre- 
sentatives out  of  the  water  to  seek  a  living  on  the  dry  land. 
Its  four  branches  (D,  E,  Fy  and  G)  now  constitute  the  entire 
population  of  the  dry  land,  as  well  as  all  the  more  active  resi- 
dents of  the  ocean.  The  arthropods  and  vertebrates  have  been 
the  most  successful  of  the  dry-land  colonists,  and  have  risen, 
both  physically  and  mentally,  far  above  their  relatives  who  still 
remain  below  the  sea  level. 

Most  of  the  dry-land  ARTHROPODS  became  parasites  of  the 
land  vegetation.1  As  a  result  of  the  struggle  for  existence  they 
developed  into  an  immense  variety  of  insect-lite.  At  the  same 
time  they  compelled  the  plants  to  protect  themselves,  and  so 
caused  the  development  of  an  equally  immense  variety  of 
flowering  plants.  But  their  complex  organisation  and  parasitic 
mode  of  life  limited  their  size  and  prevented  them  from  de- 
veloping further  than  the  bee  and  the  ant.  On  other  worlds 
the  same  causes  would  probably  result  in  similar  limitations. 

It  would  be  a  mistake  to  suppose  that  the  animal  with  the 
most  complex  organisation  is  necessarily  the  highest  and  most 
progressive  form  of  life.  The  most  progressive  animal,  both 
physically  and  mentally,  is  likely  to  be  the  one  which  has  the 

variety  of  forms  which  have  arisen  therein.     Its  development  appears  to  have 
been  checked  by  the  limitations  of  its  habitat.     Even  the  land  mammals  which 
have  gone  back  to  live  in  the  sea  have  degenerated,  both  physically  and  mentally. 
1  Some  of  them  afterward  became  parasites  on  other  animals. 


INHABITED   WORLDS  279 

very  simplest  —  and  best  arranged  —  machinery  that  is  really 
effective  in  accomplishing  the  desired  ends. 

On  our  world  the  arthropods,  with  their  many  segments  and 
numerous  limbs,  were  compelled  (like  the  fishes  in  the  sea)  to 
waste  their  energies  in  mere  multiplication  of  non-progressive 
species.  The  fish-like  VERTEBRATES,  on  the  other  hand,  with 
their  simple,  well-arranged,  and  centralised  organisation,  and 
with  the  smallest  effective  number  of  limbs,  made  rapid  prog- 
ress as  soon  as  they  had  adapted  themselves  to  living  on  the 
dry  land.  Their  gills  were  replaced  by  lungs,  and  their  lateral 
fins  developed  into  legs,  with  five  toes  on  each  foot.  The  more 
varied  conditions  to  which  they  were  exposed  in  their  new 
habitat,  combined  with  the  violent  struggle  for  existence  which 
soon  arose,  led  to  great  developments  in  size  and  strength  or  in 
nimbleness  and  cunning.  Their  internal  organs  became  more 
effective,  and  their  organs  of  sense  more  acute.  They  developed 
into  amphibians,  reptiles,  birds,  and  mammals.  Many  of  them 
became  carnivorous,  and  developed  claws  and  teeth  suitable  for 
their  bloodthirsty  profession.  This  led  to  the  survival  of  those 
vegetarian  forms  which  were  most  successful  in  defence,  con- 
cealment, or  flight.  A  great  variety  of  species  arose  from  this 
struggle  for  existence. 

Some  of  the  tree-dwelling  quadrupeds  learned  to  use  their 
feet  for  climbing,  and  for  swinging  from  tree  to  tree.  When 
they  were  subsequently  forced  to  live  on  the  treeless  plains, 
they  walked  upright  on  their  hind  legs,  and  used  their  front 
feet  for  grasping  weapons  and  tools.  Their  mental  develop- 
ment was  so  much  hastened  by  this  new  use  for  their  front 
feet  that  they  gradually  learned  to  utilise  the  forces  of  Nature 
in  addition  to  their  own  strength.  They  also  learned  to  convey 
their  thoughts  to  one  another  by  a  constantly  increasing  variety 
of  vocal  sounds,  and  mutually  to  co-operate  for  the  attainment 
of  any  desired  end.  As  they  were  now  able  to  protect  them- 
selves from  the  elements,  and  from  all  kinds  of  difficulties  and 
dangers,  they  soon  spread  from  land  to  land  without  under- 
going any  great  physical  modifications.  Their  descendants  are 


280     HOW   TO   KNOW   THE   STARRY  HEAVENS 

now  to  be  found  all  over  the  continents  and  islands  of  the 
Third  Planet,  and  the  species  is  known  to  science  by  the  name 
of  Homo  sapiens. 

Now  the  innumerable  modifications  that  have,  on  our  Earth, 
led  from  the  first  protista  to  the  latest  man,  are  entirely  the 
result  of  the  struggle  for  existence.  This  struggle  may  at  the 
very  first  have  been  entirely  physical  and  chemical.  But  when 
once  any  particular  part  of  the  World  became  crowded  with 
living  organisms,  there  arose  a  much  more  terrible  struggle, 
between  the  different  species,  and  also  between  the  individuals 
composing  those  species.  This  struggle  was  one  for  the  neces- 
saries of  life,  such  as  food,  water,  air,  and  light.  All  kinds 
of  organic  life  were  blindly  and  intensely  prolific,  but  the 
World  was  small,  and  the  necessaries  of  life  were  strictly 
limited  in  amount.  The  entire  Earth  therefore  became  like 
the  Black  Hole  of  Calcutta,  packed  with  a  struggling  mass  of 
starving  and  suffocating  organisms.  For  every  one  that  lived 
long  enough  to  propagate  its  species,  a  -thousand  perished  in 
infancy.  Only  those  that  were  strong,  savage,  nimble,  cunning, 
unscrupulous,  or  uneatable,  could  survive  and  hand  down  their 
peculiarities  to  their  posterity.  The  surviving  forms  are  there- 
fore those  which  are  best  fitted  to  carry  on  the  struggle. 

The  physical  and  mental  peculiarities  which  have  been  most 
successful  here  would  probably  be  most  successful  in  a  similar 
struggle  for  existence  on  any  other  world.  And  the  course  of 
organic  evolution  would  therefore  be  somewhat  similar.  Hence 
we  may  say,  with  George  Morris : 

"  There  is  considerable  reason  to  believe  that  the  beings  which 
answer  to  man  upon  any  of  the  planets  of  the  Universe  must  at  least 
approach  man  somewhat  closely  in  physical  configuration.  ...  It 
certainly  seems  as  if  a  human  traveller,  if  he  could  make  a  tour  of 
the  Universe,  would  find  beings  whom  he  could  hail  as  kindred  upon 
a  thousand  spheres."  * 

1  Those  who  wish  to  follow  up  this  subject  should  read  an  article  by  Mr. 
Morris  in  the  Popular  Science  Monthly  for  April,  1904. 


INHABITED   WORLDS  281 


A  POET'S  DREAM 

The  astronomer,  as  such,  has  nothing  to  say  of  the  unknown 
citizens  of  these  unknown  worlds.  Accustomed,  as  he  is,  to 
test  his  theories  by  observation,  he  holds  lightly  to  unprovable 
speculations,  however  probable  they  may  be. 

The  speculative  poet,  however,  trusts  himself  in  airy  flights 
that  extend  far  beyond  the  limits  of  the  known.  He  not  only 
wrestles  boldly  with  half-seen  facts,  but  also  concerns  himself 
with  invisible  probabilities.  With  an  imagination  that  knows 
no  fetters  except  those  of  natural  law,  he  sees,  beyond  the 
utmost  range  of  telescope  and  camera,  the  — 

—  "  fire  of  unrecorded  stars 

That  light  a  heaven  not  our  own." 

He  beholds,  with  a  certainty  that  is  based  on  mathematics 

and  physics  — 

—  "the  hidden  gyre 
Of  bulks  that  strain  in  Algol's  toils." 

He  glimpses,  with  a  probability  that  is  born  of  his  earthly 
observations,  the  — 

—  "  seas  that  flash  on  alien  eyes 
The  riven  sunlight  of  Altair." 

In  many  a  far-off  globe  he  sees  a  dawn  of  life,  followed  by  a 
physical  and  mental  evolution  which  culminates  in  the  develop- 
ment of  reasoning  beings.  On  many  an  unknown  world  he 
watches  an  age  of  faith  slowly  change  to  an  age  of  reason.  On 
many  a  sun-kissed  planet  he  hears  infantile  stories  of  a  one- 
world  creation  give  way  to  more  mature  discussions  of  a  vast 
and  abiding  Universe.  He  imagines  these  unknown  phi- 
losophers discussing  the  whence,  the  where,  the  what,  the 
why,  and  the  whither  of  all  things  —  even  as  some  of  us  do 
here. 

He  sees  these  lengthy  discussions  brought  to  a  futile  close  by 
the  gathering  darkness  and  cold  of  everlasting  night  —  to  be  as 


282     HOW  TO   KNOW  THE   STARRY   HEAVENS 

fruitlessly  brought  up,  again  and  again,  on  other  worlds,  in 
systems  yet  to  come.  For  the  great  "  Eiddle  of  the  Universe  " 
can  never  be  fully  solved  by  finite  minds.  As  Olive  Schreiner 
says  of  the  drama  that  is  being  enacted  on  our  own  Earth  : 

"  What  the  name  of  the  play  is,  no  one  knows.  If  there  sits  a  spec- 
tator who  knows,  he  sits  so  high  that  the  players  in  the  gaslight 
cannot  hear  his  breathing." 

George  Sterling,  in  his  immortal  "  Testimony  of  the  Suns," 

"  So  dreamt  thy  sons  on  worlds  destroyed, 
Whose  dust  allures  our  careless  eyes, 
As,  lit  at  last  on  alien  skies, 
The  meteor  melts  athwart  the  void. 

"  So  shall  thy  seed  on  worlds  to  be, 
At  altars  built  to  suns  afar, 
Crave  from  the  silence  of  the  star 
Solution  of  thy  mystery. 

"  And  crave  unanswered,  till,  denied 
By  cosmic  gloom  and  stellar  glare, 
The  brains  are  dust  that  bore  the  prayer, 
And  dust  the  yearning  lips  that  cried." 


CHAPTER  XXIV 

SIZE,  IMPORTANCE,   SPEED,  AND  DURATION 

SIZE  AND  IMPORTANCE  RELATIVE 

"  The  appearance  of  things  depends  altogether  on  the  point  of  view  we  oc- 
cupy. He  who  is  immersed  in  the  turmoil  of  a  crowded  city  sees  nothing  but  the 
acts  of  men.  .  .  .  But  he  who  ascends  to  a  sufficient  elevation  .  .  .  discovers  that 
the  importance  of  individual  action  is  diminishing  as  the  panorama  beneath  him 
is  extending.  And  if  he  could  attain  to  the  truly  philosophical,  the  general  point 
of  view,  .  .  .  rising  high  enough  to  see  the  whole  world  at  a  glance,  his  acutest 
vision  would  fail  to  discover  the  slightest  indication  of  man,  his  free  will,  or  his 
works.  In  her  resistless  onward  sweep,  in  the  clock-like  precision  of  her  daily 
and  nightly  revolution,  in  the  well-known  pictured  forms  of  her  continents  and 
seas,  now  no  longer  dark  and  doubtful,  but  shedding  forth  a  planetary  light,  well 
might  he  ask  what  had  become  of  all  the  aspirations  and  anxieties,  the  pleasure 
and  agony  of  life.  As  the  voluntary  vanished  from  his  sight,  and  the  irresistible 
remained,  well  might  he  incline  to  question  .  .  .  whether  beneath  the  vast- 
ness,  energy,  and  immutable  course  of  a  moving  world  there  lay  concealed  the 
feebleness  and  imbecility  of  man.  Yet  it  is  none  the  less  true  that  these  contra- 
dictory conditions  co-exist  —  Free-will  and  Fate,  Uncertainty  and.  Destiny.  It  is 
only  the  point  of  view  that  has  changed,  but  on  that  how  much  has  depended." 
—  Dr.  J.  W.  Draper. 

THE  human  family  is  an  ephemeral  form  of  organic  life 
entirely  confined  to  one  insignificant  planet  in  an  incon- 
spicuous stellar  system.  And  the  still  more  ephemeral  individ- 
uals composing  it  are  generally  cooped  up  in  a  small  corner  of 
this  little  planet.  It  is  therefore  very  difficult  for  those  who 
take  an  interest  in  cosmical  affairs  to  get  correct  and  undistorted 
ideas  as  to  the  relative  sizes  and  importance  of  the  objects  com- 
posing the  Universe.  Nor  is  it  easy  for  them  to  accept  the 
teachings  of  Astronomy  with  regard  to  the  enormous  speed  with 
which  some  of  these  objects  move  through  space,  or  the  im- 
mense duration  of  the  heavenly  bodies  in  general.  It  may  not, 


284    HOW  TO   KNOW  THE   STARRY  HEAVENS 

therefore,  be  out  of  place  to  conclude  our  bird's-eye  examination 
of  the  Universe  with  a  few  words  on  these  subjects. 

Man  is  a  very  important  personage  —  in  his  own  [estimation. 
As  a  general  thing  he  is  so  wrapped  up  in  the  petty  details  of 
his  own  mundane  existence,  that  he  fails  to  realise  his  own 
insignificance,  either  in  the  Universe  at  large  or  in  the  little 
World  on  which  he  lives  and  moves  and  has  his  being. 

Yet  a  man  is  only  one  out  of  some  1,400,000,000  of  similar 
beings.  And  as  knowledge  and  wisdom  bring  modesty,  he  who 
thinks  himself  better  and  more  important  than  his  fellows  is 
generally  of  very  little  account.  t 

The  same  is  true  of  the  entire  living  race.  All  the  men, 
women,  and  children  on  Earth  are  but  a  handful  to  those  who 
have  gone  before,  and  will  come  hereafter  in  the  ages  yet  to 
come.  Future  generations  will  look  back  with  pity  on  our 
boasted  wisdom  and  knowledge,  even  as  we  do  on  the  wisdom 
and  knowledge  of  former  times. 

The  human  race,  as  a  whole,  believes  itself  to  be  the  raison 
d'etre  of  the  Universe,  — "  the  end  and  aim  of  all  creation." 
Yet  it  is  but  a  thing  of  yesterday,  doomed  to  perish  to-morrow 
and  be  forgotten.  For  millions  of  years  the  Earth  spun  around 
its  reeling  axis,  and  circled  around  its  little  twinkling  star,  in 
company  with  a  crowd  of  other  planets.  It  did  all  this  without 
the  presence  of  man,  and  it  will  continue  to  circle  and  spin 
when  the  human  family  has  passed  away,  like  a  streak  of  morn- 
ing cloud,  into  the  infinite  azure  of  the  past. 

Our  World  is  a  very  mighty  world  —  to  us.  How  mighty  it 
is,  only  those  who  have  traversed  its  oceans  and  continents  can 
truly  realise. 

Yet,  compared  with  the  rest  of  the  Universe,  it  is  very,  very 
small. 

So  small  is  it  that  the  mind  cannot  grasp  the  idea  of  its 
littleness. 


SIZE,  IMPORTANCE,  SPEED,  AND   DURATION     285 

It  is  as  a  single  leaf,  compared  with  all  the  leaves  of  the 
forest. 

It  is  as  a  blade  of  grass  that  withereth  away,  compared  with 
all  the  grasses  of  the  World. 

It  is  as  a  drop  of  water,  compared  with  all  the  oceans  and 
seas. 

It  is  as  a  grain  of  sand,  compared  with  a  world  of  similar 
grains. 

Our  Sun  is  1,250,000  times  as  large  as  our  Earth,  and  con- 
tains 330,000  times  as  much  matter. 

Though  it  is  more  than  90,000,000  miles  away,  it  is  bright 
enough  to  make  the  electric  light  seem  black  by  contrast. 

It  is  hot  enough  to  turn  our  entire  Earth  into  fire-mist  if  it 
were  dropped  into  it.  It  is  powerful  enough  to  keep  the  planet 
Neptune  in  bondage,  although  thirty  times  as  far  from  it  as 
our  Earth. 

Yet  this  mighty  Sun  of  ours  is  only  one  out  of  millions  of 
similar  suns.  It  is  only  a  star  among  a  Universe  of  stars. 

Space  is  endless  —  limitless  —  bottomless.  It  has  no  bounds, 
either  visible  or  invisible.  It  is  a  sea  without  surface  or 
bottom,  an  ocean  without  a  shore.  And  our  telescopes  and 
spectroscopes  show  that,  all  around  us  in  the  depths  of  this 
shoreless  ocean,  living  suns  like  ours  are  strewed  in  hundreds 
of  millions,  while  nebulae  and  dead  suns  exist  beyond  all  com- 
putation. 

SPEED  IS  RELATIVE 

When  we  hear  that  our  World  goes  around  the  Sun  at  the 
speed  of  18  J  miles  in  a  second  of  time,  and  that  the  Solar 
System  itself  is  speeding  toward  the  constellation  of  Lyra  at 
the  rate  of  12 1  miles  per  second,  we  naturally  compare  such 
motions  with  the  rush  of  a  bullet  or  cannon-ball.  The  result 
of  the  comparison  is  that  the  statements  appear  almost  unbe- 
lievable. But  if  we  change  the  comparison,  and  note  that  the 
Earth  is  seven  minutes  in  moving  the  length  of  her  own  diam- 


286     HOW   TO   KNOW  THE   STARRY  HEAVENS 

eter,  and  that  the  Sun  is  16  hours  in  doing  the  same  thing,  the 
celestial  velocities  seem  very  moderate.  The  first  comparison 
calls  to  mind  the  sharp  "  ping  "  of  an  invisible  projectile.  The 
second  reminds  one  of  the  stately  and  almost  imperceptible 
drift  of  a  family  of  icebergs  through  the  arctic  seas. 

In  this  case  the  figures  do  not  become  smaller,  but  the  stand- 
point is  changed,  and  that  makes  a  wonderful  difference  in  the 
result. 

It  is  true  that  astronomy  shows  us  instances  of  extremely 
small  bodies  travelling  at  even  greater  speed  than  the  large 
ones.  But  in  these  cases  it  will  be  found  that  the  small  bodies 
are  not  moving  by  any  energy  of  their  own,  or  by  any  initial 
force  that  soon  ceases  to  act.  They  are  under  the  continuous 
influence  of  some  celestial  giant  whose  power  is  almost  im- 
measurably great.  The  energy  exerted  is  not  merely  initial 
but  continuous  and  cumulative.  The  push  or  pull  is  therefore 
unimaginably  vast,  compared  with  any  force  that  man  can 
bring  to  bear  on  such  an  object.  This  being  the  case,  it  is  quite 
natural  and  reasonable  that  the  resulting  motions  should  be 
almost  beyond  his  comprehension  and  belief. 

DURATION  IS  RELATIVE 

"  A  generation  passeth  away, 
And  a  generation  cometh, 
But  the  Earth  abideth 
For  ages  of  ages.  —  Eccles.  i,  4  (A.  Zazel). 

"  But  yestermorn,  O  boastful  clay, 

Thy  planet  from  its  parent  broke  — 
O,  insect  of  a  summer's  day, 

Thy  vaunted  glories  pass  like  smoke  !  "  —  George  N.  Lowe. 

To  some  low  forms  of  animal  life  a  day  represents  an  entire 
lifetime.  In  one  day  an  entire  generation  is  born,  flourishes, 
and  dies.1 

1  In  some  of  the  single-celled  monera  the  average  life  of  an  individual  is  about 
four  minutes,  so  that  there  are  360  generations  in  a  day  of  24  hours.  One  of  our 
days  is  therefore  equal  to  12,000  years  with  them. 


SIZE,  IMPORTANCE,  SPEED,  AND  DURATION     287 

Yet,  to  a  man,  a  day  seems  a  very  short  period  of  time  — 
that  gets  still  shorter  as  he  advances  in  years. 

To  a  youth  who  is  just  entering  into  life,  seventy  years  seem 
to  make  an  almost  endless  stretch  of  time.  There  appear  to  be 
hardly  any  limits  to  what  may  be  accomplished  in  threescore- 
and-ten  long,  long  years. 

But  to  the  old  man  that  life  is  little  more  than  a  dream  — 

of  high  hopes  unfulfilled  —  of  noble  aspirations  that  have  been 

abandoned  —  of  great  expectations  that  have  come  to  nought 

—  of  tardy   realisations  that    have   turned    to   ashes   in  the 

mouth. 

In  the  lifetime  of  one  individual  many  events  take  place 
among  men,  many  changes  occur  in  the  World  on  which  he 
lives. 

Yet  all  the  world-wide  occurrences  during  such  a  lifetime 
form  but  a  page  of  that  history  which  —  in  Egypt  and  elsewhere 
—  goes  back  more  than  6,000  years. 

Six  thousand  years  of  human  activities  almost  becloud  the 
mind.  Through  the  mists  of  antiquity  the  early  actors  loom 
vague  and  vast.  To  the  unenlightened  they  seem  as  Gods  who 
wield  the  thunderbolt,  control  the  tempest,  and  forge  the  chains 
of  human  destiny. 

Yet  6,000  years  form  but  a  fragment  of  the  history  of  man 
as  revealed  by  the  study  of  his  remains.  The  human  family  is 
very,  very  ancient.  So  ancient  indeed  is  it  that  many  people 
refuse  to  accept  the  evidence  they  cannot  overthrow. 

If  we  assume  that  6,000  generations  of  men  have  lived  on 
this  Earth  —  and  the  time  cannot  have  been  much  less  than 
that  —  we  are  utterly  bewildered  when  we  endeavour  to  realise 
such  an  abyss  of  time. 

Yet  such  an  interval  is  but  a  watch  in  the  night  when  we 
compare  it  with  the  vast  geological  periods  intervening  between 
the  dawn  of  life  on  Earth  and  the  days  when  man  first  stood 
erect  on  terra  firma.  It  must  have  taken  millions  of  years  to 
deposit  the  Carboniferous  rocks  alone,  and  they  represent  but  a 
small  fragment  of  geologic  time. 


288     HOW  TO   KNOW  THE   STARRY   HEAVENS 

Before  the  dawn  of  life  on  this  globe  there  was  a  period 
during  which  the  first  gaseous  and  then  molten  Earth  was  but 
"  a  bare  lurid  ball  in  the  vast  wilds  of  space."  During  fathom- 
less ages  it  slowly  cooled  off,  till  at  last  the  surface  became 
solid,  and  the  vaporous  ocean  above  was  no  longer  kept  off  the 
blistering  surface. 

How  long  that  period  lasted  we  are  utterly  ignorant.  For  all 
we  know  it  may  have  exceeded  geological  times  as  geological 
times  exceeded  the  era  of  man. 

Before  the  Earth  was  born  —  as  a  gaseous  centre  of  conden- 
sation in  a  swirling  mist,  subsequently  to  shrink  into  a  fiery 
globe  —  was  a  long,  long  period,  during  which  a  vast  and  shape- 
less nebula  turned  into  a  flat  and  symmetrical  indrawing  spiral. 

And  before  that  shapeless  nebula  was,  the  Universe  is. 

As  E.  A.  Proctor  says : 

"  The  whole  duration  of  this  Earth's  existence  is  but  as  a  single 
pulsation  in  the  mighty  life  of  the  Univer.se.  Nay,  the  duration  of 
the  Solar  System  is  scarcely  more.  Countless  other  such  systems 
have  passed  through  all  their  stages,  and  have  died  out,  untold  ages 
before  the  Sun  and  his  family  began  to  be  formed  out  of  their  mighty 
nebula;  countless  others  will  come  into  being  after  the  life  has 
departed  from  our  system.  Nor  need  we  stop  at  solar  systems,  since 
within  the  infinite  Universe,  without  beginning  and  without  end,  not 
suns  only,  but  systems  of  suns,  galaxies  of  such  systems,  to  higher 
and  higher  orders  endlessly,  have  long  since  passed  through  all  the 
stages  of  their  existence  as  systems,  or  have  all  these  stages  yet  to 
pass  through."  • 

It  appears  certain  that  matter  and  energy  always  existed  and 
always  will  exist.  They  were  never  created  and  will  never  be 
destroyed.  We  have  reason  to  believe  that  through  all  time 
the  Universe  contains  exactly  the  same  number  of  corpuscles, 
the  same  amount  of  energy.  Suns  and  worlds  come  and  go, 
like  bubbles,  on  the  Eiver  of  Eternity.  But  the  matter  of 
which  they  are  composed  endures  for  ever  and  ever.  The 
energy  they  contain  neither  waxes  nor  wanes.  The  one  and 
the  other  are  eternal  —  everlasting  —  imperishable.  Science 


SIZE,  IMPORTANCE,  SPEED,  AND  DURATION     289 

teaches  us  that  though  the  heavens  change  and  the  worlds 
decay,  neither  a  corpuscle  of  matter  nor  an  iota  of  energy  will 
ever  cease  to  exist. 

SUMMARY 

We  thus  see  that  size,  importance,  speed,  and  duration  are 
all  relative  terms,  denoting  attributes  that  are  either  great  or 
small  according  to  the  standpoint  from  which  they  are  viewed. 

The  object  of  this  work  is  to  enable  us  to  view  all  subjects 
from  as  many  different  standpoints  as  possible,  so  that  we  may 
not  be  deceived  in  the  conclusions  at  which  we  arrive,  about 
the  Universe  in  general  and  our  World  in  particular ;  about  the 
human  race  as  a  grand  total,  and  an  individual  as  a  minute 
fraction  of  it;  about  Eternity  as  a  limitless  whole,  and  our 
Earth-measured  years  as  microscopic  parts  thereof. 


19 


CHAPTER  XXY 

CONCLUSION 

"An  undercut  astronomer  is  mad."  —  E.  Young.  • 

"  Most  of  the  religions  of  the  world  are  more  or  less  derived  from  Astronomy 
in  its  astrological  childhood.  The  majority  of  their  deities  were  once  identified 
with  the  sun,  moon,  planets,  or  stars.  Their  temples,  vestments,  feasts,  fasts, 
ceremonies,  and  writings,  contain  multitudes  of  celestial  fossils  which  only  astrono- 
mers can  recognise  and  explain."  —  A.  Zazel. 

"It  is  Astronomy  which  will  eventually  be  the  chief  educator  and  emancipator 
of  the  race."  —  Sir  Edwin  Arnold. 

I  HAVE  now  finished  an  account  that  is  little  more  than  a  bare 
outline  of  what  Science  has  to  say  about  the  Universe.     Is 
it  not  a  wonderful  story,  even  when  poorly  told  ?     Is  it  not  a 
sublime  picture,  though  seen  as  through  a  glass,  darkly  ? 

The  astronomer  of  to-day  is  a  cosmopolitan  in  the  widest 
sense  of  the  word.  He  is  a  citizen  of  the  Great  Cosmos  —  a 
traveller  on  the  Ocean  of  Space  —  an  explorer  of  what  was  but 
yesterday  an  unknown  Universe.  Though  he  is  physically  a 
prisoner  on  Earth,  he  is  mentally  free  to  roam  through  all  the 
mansions  of  the  skies.  Though  he  is  but  one  of  a  multitude 
of  world-mites,  he  is  the  privileged  spectator  of  the  greatest  of 
all  the  dramas.  Though  he  is  but  an  ephemeron,  he  can  watch 
the  mystic  whirling  of  the  myriad  worlds  through  endless  time 
and  space.  In  the  grandest  of  all  temples  he  can  hear  the 
sublime  "  music  of  the  spheres "  echoing  for  ever  through 
the  lofty  aisles.  From  his  celestial  watch-towers  he  surveys 
the  wonders  of  "  the  house  not  made  with  hands,  eternal  in  the 
heavens."  He  stands  before  the  INFINITE  ;  he  contemplates  the 
ETERNAL. 

The  human  race  owes  much  more  to  astronomy  and  its  kindred 
sciences  than  it  is  aware  of.  All  the  production,  transportation, 


CONCLUSION  291 

and  distribution  of  the  necessaries  of  life  are  regulated  by  the 
clock  of  the  astronomer.  Every  train  that  crosses  the  conti- 
nents with  speed  and  safety  does  it  through  his  observations. 
Every  ship  that  crosses  the  otherwise  trackless  oceans  is  guided 
to  port  by  his  instruments  and  calculations.  His  predictions 
of  astronomical  phenomena  are  published  in  the  "Nautical 
Almanac "  several  years  before  they  occur,  so  that  navigators 
may  be  able  to  correct  the  errors  of  their  chronometers  in 
whatever  part  of  the  World  they  may  happen  to  be. 

A  single  mistake  in  these  predictions  may  bring  death  and 
disaster  in  any  part  of  the  globe.  An  improved  method  of  cal- 
culation or  observation  may  bring  increased  speed,  certainty, 
and  safety  to  the  commerce  of  all  the  World. 

The  science  of  human  history  is  largely  indebted  to  astron- 
omy for  the  correction  of  its  dates.  .For  some  of  the  eclipses, 
comets,  etc.,  which  are  mentioned  in  ancient  documents  and 
traditions,  have  been  identified  by  calculating  back,  and  the 
exact  dates  have  been  ascertained.  When  these  have  once 
been  fixed,  all  the  neighbouring  dates  can  be  adjusted  to  those 
which  have  been  ascertained  astronomically. 

Some  departments  of  science  appear,  at  first  sight,  to  be 
practically  useless,  because  they  have,  or  appear  to  have,  no 
direct  bearing  on  the  welfare  of  the  race.  Yet  even  these  bring 
indirect  benefits  which  are  too  vast  to  be  easily  realised.  Pure 
science  can  be  cultivated  only  for  itself,  but  it  forms  the  foun- 
dation on  which  all  practical  science  is  based.  Then  the  les- 
sons that  science  teaches,  on  the  art  of  getting  at  the  truth,  are 
as  valuable  as  its  most  practical  discoveries  and  practicable 
inventions.  This  is  especially  the  case  with  astronomy.  Even 
the  necessary  ills  of  life  are  lessened  by  a  knowledge  of  the 
heavens,  for  in  the  presence  of  the  illimitable  and  eternal  Uni- 
verse it  seems  absurd  to  worry  about  the  little  earthly  troubles 
that  we  cannot  remedy.  We  learn  to  accept  the  inevitable  and 
make  the  best  of  it. 

It  may  indeed  be  truthfully  said  that  a  fair  knowledge  of 
astronomy  and  geology  will  double  the  pleasures  of  life  and 


292     HOW  TO   KNOW  THE   STARRY   HEAVENS 

halve  its  troubles.     At  the  same  time  it  will  entirely  remove 
the  creed-made  terrors  of  death. 

There  are  not  many  of  us  who  can  do  much  toward  gaining 
fresh  knowledge  of  the  secrets  of  Nature.  But  there  is  no 
reason  why  we  should  not  enlighten,  broaden,  and  refresh  our- 
selves by  taking  an  interest  in  the  scientific  discoveries  which 
are  revolutionising  the  world  and  revealing  the  Universe  to 
man. 

The  study  of  the  Universe  will  not  only  make  us  enlightened 
citizens  of  the  Great  Cosmos,  but  will  also  make  us  better  citi- 
zens of  our  own  World.  It  will  do  this  by  teaching  us  to  observe 
correctly  the  surrounding  social  and  economic  phenomena, 
instead  of  blindly  trusting  to  the  assertions  of  prejudiced  and 
perhaps  interested  persons.  It  will  also  train  us  to  reason  cor- 
rectly and  impartially  as  to  the  causes  and  effects  of  the 
observed  phenomena.  We  find  ourselves,  at  the  commence- 
ment of  the  twentieth  century,  confronted  by  serious  industrial, 
economic,  and  social  problems  which  demand  solution  at  our 
hands.  These  problems  are  largely  the  result  of  scientific 
instruments  of  production  and  distribution,  invented  and 
developed  during  the  last  two  centuries.  We  cannot  solve 
these  problems  satisfactorily  and  equitably  till  we  recognise 
the  fact  that  human  progress  is  subject  to  evolution  —  that  it 
is  in  fact  ruled  by  natural  laws  just  as  inexorable  as  those 
which  govern  suns  and  worlds.  It  is  therefore  necessary  for 
us  to  ascertain  what  these  laws  are,  and  then  to  solve  our  in- 
dustrial and  social  problems  in  accordance  with  them.  When 
this  is  done,  our  wonderful  advance  in  the  arts  and  sciences 
will  be  paralleled  by  as  rapid  an  advance  toward  universal 
prosperity  and  happiness.  But  not  before. 

NOTE.  —  Those  who  wish  to  continue  the  study  of  the  starry  heavens  will  do 
well  to  get  the  popular  works  of  Airy,  Ball,  and  Serviss;  also  Comstock's 
"  Text-book  of  Astronomy,"  which,  at  its  close,  gives  a  list  of  some  of  the  best 
works  on  astronomy,  both  general  and  special. 


CONCLUSION  293 

ASTRONOMY 

BY  GEORGE  N.  LOWE 

(By  permission') 

Great  Antidote  for  ignorance  and  fears, 

We  look  to  thee  to  lead  us  to  the  day. 

Before  thy  light  the  darkness  melts  away  — 
Thy  ceaseless  labour  Life's  great  pathway  clears. 
The  shackles  fall  as  pass  the  stately  years, 

Thy  gaze  uplifts,  lo  !     Yega's  distant  day 

Comes,  captive,  where  thy  mystic  colours  play  — 
The  sombre  gloom  of  ages  disappears. 

What  though  the  thrones  of  false  gods  shake  and  fall, 

Though  useless,  hoary,  man-made  creeds  may  fade  ? 

Calm  Reason  knows  no  sorrow,  reck,  or  ruth. 

Thy  touch,  Boon  Science,  liberates  the  thrall ; 

Thy  broadening  beam  illumes  the  unknown  shade  ; 

Patient,  thy  finger  pointeth  to  the  Truth. 


APPENDIX  A 

FACTS  AND  FANCIES  CONCERNING  MATTER 

"  What  is  Matter  ?     Never  mind. 
What  is  Mind  ?     No  matter."  —  Old  Queries. 

"  What  is  Matter  ?     Only  a  cloud  of  Corpuscles. 
What  are  Corpuscles  ?     Only  Electricity. 
What  is  Electricity  ?     Only  Matter. 
What  are  they  all  ?     No  matter.     Never  mind  ! " —  New  Queries. 

ONLY  A  ROCK 

TAKE  up  a  small  rock  in  your  hand  and  examine  it  care- 
fully with  the  naked  eye. 

It  appears  to  consist  of  a  number  of  irregular  granules,  dif- 
fering in  size  and  shape,  but  alike  in  colour  and  texture.  It  is 
only  a  piece  of  rock,  rough,  heavy,  more  or  less  hard  and  com- 
pact, and  altogether  insignificant. 

Such  a  rock  might  serve  a  teamster  very  well  to  throw  at 
his  off  leader,  or  a  small  boy  might  use  it  as  a  nucleus  for  a 
vicious  snow-ball,  but  otherwise  it  is  of  no  value  to  man  or 
beast. 

TWO  STANDPOINTS 

Yet  these  ideas  concerning  its  insignificance  are  all  owing 
to  the  fact  that  we  view  it  from  a  human  standpoint,  without 
even  the  aid  of  science. 

If  we  could  examine  it  from  the  standpoint  of  one  who 
possessed  the  all-seeing  eye  popularly  attributed  to  some  of  the 
Gods,  we  should  perhaps  come  to  very  different  conclusions 
concerning  its  size,  condition,  and  importance. 


296     HOW   TO   KNOW  THE   STARRY  HEAVENS 

In  order  to  do  this,  we  shall  have  to  increase  our  powers  of 
vision  and  analysis  by  resorting  to  the  microscope  and  other 
scientific  instruments. 

A  MAGNIFIED  ROCK 

On  applying  a  powerful  magnifying-glass  to  the  rock,  we 
find  that  it  no  longer  appears  small  and  insignificant.  In  fact 
it  is  now  so  large  that  we  can  examine  only  a  small  part  of  it 
at  once. 

Its  granulated  surface  has  swelled  into  rugged  rock-masses 
divided  by  narrow  gulches  and  clefts.  The  surface  of  these 
masses  of  rock  has  the  same  granulated  appearance  that  the 
whole  rock  had  to  the  naked  eye. 

We  will  now  select  the  smallest  visible  speck,  and  examine 
it  with  the  microscope.  The  result  is  that  the  speck,  which 
was  almost  invisible  with  the  single  magnifying-glass,  is  now 
a  huge  mass  of  rocky  hills  separated  by  gloomy  gorges. 

On  closely  observing  the  rocks  which  compose  these  hills,  we 
see  that  the  whole  mass  is  honeycombed  with  crevices  which 
were  invisible  before.  Each  mighty  rock  is  seen  to  be  com- 
posed of  small  granules  loosely  connected  together. 

A  WORLD  IN  ITSELF 

By  vastly  increasing  the  power  of  our  microscope  the  hills 
become  mountain-chains  whose  lofty  peaks  extend  away  into 
the  remote  distance.  The  stone  which  could  be  held  in  the 
hand  has  become  a  world  in  itself,  shutting  out  the  firmament 
from  view. 

Again  and  again  we  select  the  smallest  visible  granule,  and 
subject  it  to  examination  with  higher  magnifying  power. 
Each  time  we  do  so  the  almost  invisible  particle  selected  swells 
into  an  enormous  mass  that  fills  the  field  of  view.  Each  suc- 
cessive particle  is  seen  to  consist  of  hills  and  gorges  of  more 
or  less  solid  rock,  composed  of  scarcely  visible  granules.  But 
the  substance  of  these  granules  is  still  the  same  as  that  com- 
posing the  whole  rock. 


FACTS  AND  FANCIES  CONCERNING  MATTER     297 

The  magnifying  powers  of  the  most  powerful  microscope  in 
existence  are  at  last  exhausted.  We  have  to  continue  our 
examination  by  means  of  the  spectroscope  and  other  instru- 
ments, and  view  the  results  of  our  investigations  with  the  eye 
of  the  imagination. 

Confining  ourselves  to  the  smallest  sub-granule  revealed  by 
the  most  powerful  microscope,  we  see  it  gradually  unfold  itself 
as  the  magnifying  power  of  the  imagination  is  applied  to  it. 
When  it  becomes  too  large  for  minute  examination,  we  select 
an  almost  invisible  particle  of  it,  and  watch  it  grow  till  it  fills 
the  entire  field  of  view.  Again  and  again  we  do  the  same 
thing,  each  successive  particle  getting  too  large  for  inspection 
as  it  swells  under  the  scrutiny  of  the  scientific  imagination. 

As  this  process  goes  on,  we  notice  a  progressive  change,  not 
in  the  substance,  but  in  the  structure  of  what  we  are  examining. 
It  slowly  loses  its  original  solid  appearance  and  becomes  a 
dense  nebulous  haze. 

MOLECULES 

This  haze  gradually  opens  out,  and  finally  resolves  itself  into 
a  universe  of  small  separate  bodies  which  are  commonly  known 
as  molecules.  These  appear  to  float  in  empty  space  without 
touching  one  another. 

ATOMS 

As  the  magnifying  power  of  our  imagination  goes  on  increas- 
ing, each  molecule  is  seen  to  be  a  group  of  shining  atoms, 
clinging  together  in  "  radical "  bunches,  yet  avoiding  actual 
contact.  These  atoms  are  altogether  different  in  appearance 
and  properties  from  the  molecules  which  they  compose.  They 
may  be  sorted  out  into  several  different  species,  the  individual 
atoms  of  each  kind  being  all  absolutely  alike,  but  different  in 
size,  weight,  and  other  particulars  from  those  of  the  other 
species. 

CAPTIVE  CORPUSCLES 

The  question  now  arises,  What  are  these  different  kinds  of 
atoms  which  constitute  the  molecules  of  which  our  rock  is 


298    HOW  TO   KNOW  THE   STARRY   HEAVENS 

built  up  ?  Are  they  single  and  indivisible,  independent  and 
unrelated,  or  are  they,  too,  composed  of  still  smaller  particles? 

In  order  to  decide  this  question  we  pick  out  a  single  atom  in 
one  of  the  molecules  and  confine  our  attention  to  it  exclusively. 
At  the  same  time  we  once  more  increase  the  separating  power 
of  the  imagination. 

The  atom  now  loses  its  indivisible  character  and  is  seen  to 
consist  of  a  star-cluster  of  minute  corpuscles.  These  appear  to 
have  a  regular  orbital  motion,  at  planetary  distances,  around 
their  centre  of  gravity.  They  are  kept  together,  and  yet 
separate,  by  a  combination  of  forces  which  is  not  yet  under- 
stood. 

These  sub-elementary  corpuscles  are  all  identical  in  form, 
weight,  and  attributes.  The  only  suspicion  of  difference  is  that 
they  act  as  though  one  half  of  them  were  positively,  and  the 
other  half  negatively,  electrified.  They  seem  to  cling  together 
in  pairs  and  form  alternate  layers  around  the  centre  of  the 
cluster,  the  layers  being  far  apart  and  sparsely  occupied. 

We  now  leave  the  atom  we  have  just  resolved  into  corpuscles, 
and  devote  our  attention  to  the  other  species  of  atoms  which 
build  up  the  molecules  of  matter.  They  are  all  resolvable  into 
star-clusters  of  corpuscles  which  are  exactly  the  same  as  those 
which  composed  the  atom  first  examined.  The  only  difference 
appears  to  be  in  the  number  and  arrangement  of  the  corpuscles 
which  compose  the  different  kinds  of  atoms.  The  smallest 
known  atoms  contain  about  a  thousand  corpuscles. 

The  outside  layer  of  corpuscles  in  an  atom  appears  to  be 
always  composed  of  negatively  electrified  corpuscles,  some  of 
which  have  the  power  of  flitting  from  one  atom  to  another, 
and  of  flying  off  into  space  as  free  and  independent  corpuscles. 

FREE  CORPUSCLES 

Besides  the  corpuscles  which  go  to  build  up  the  various 
atoms,  there  is  an  immense  multitude  of  negatively  electrified 
corpuscles  which  exist  in  a  free  state.  The  atoms  of  matter 


FACTS  AND  FANCIES  CONCERNING  MATTER     299 

move  through  this  "  dust-cloud  "  of  free  corpuscles  as  readily  as 
a  sieve  moves  through  water  without  displacing  it.  The  other- 
wise unoccupied  space  between  the  atoms  of  matter  is  filled  to 
saturation  with  these  free  negative  particles,  which  carry  the 
vibrations  of  ponderable  matter  from  one  atom  to  another. 
They  form  the  luminiferous  ether  which  fills  all  space. 

ULTIMATE  PARTICLES 

We  now  seem  to  have  at  last  reached  a  stage  where  all 
matter  is  identical  in  substance,  form,  weight,  appearance,  and 
properties.  The  ultimate  particles  of  which  matter  is  composed 
appear  to  be  revealed  to  the  eyes  of  the  imagination.  But  we 
are  still  entirely  in  the  dark  as  to  the  nature  of  these  ultimate 
and  identical  particles,  and  the  reason  why  they  group  them- 
selves as  they  do,  to  build  up  the  atoms  and  molecules  of 
ponderable  matter.  We  are  now  face  to  face  with  the  unsolved 
Riddle  of  the  Universe.  We  are  dealing  with  the  mysterious 
astronomy  of  the  atoms.  We  have  found  the  simple  bricks  out 
of  which  the  elaborate  structures  of  the  Universe  are  built,  but 
cannot  tell  whence,  nor  why,  nor  what  they  are.  Even  the 
imagination  has  at  last  reached  the  limit  of  its  power,  and  we 
are  hopelessly  lost  in  trying  to  solve  the  great  Problem  of 
Substance. 

A  TINY  UNIVERSE 

Now,  for  all  we  know  to  the  contrary,  the  nebulous  cloud  of 
matter  we  have  been  examining  may  be  a  Universe  like  ours. 
Its  molecules  may  be  aggregations  of  atomic  star-clusters  like 
those  revealed  by  the  telescope.  Its  electrical  corpuscles  may 
be  suns  like  ours,  sending  forth  their  radiant  energy  to  invisible 
worlds,  some  of  which  may  be  inhabited  by  unreasoning 
creatures  like  man. 

If  such  should  be  the  case  (and  it  would  not  be  very  easy  to 
disprove  it),  then  an  inhabitant  of  one  of  these  invisible  worlds 
probably  thinks  that  his  Earth  is  the  only  one  in  existence ; 


300     HOW  TO   KNOW  THE   STARRY  HEAVENS 

that  he  is  the  one  for  whom  all  things  were  created ;  that  his 
eternal  well-being  is  the  chief  concern  of  the  Creator ;  and  that 
his  moment  of  life  is  a  big  part  of  Eternity. 

So  much  for  the  rock  we  have  been  studying.  Let  us  now 
throw  it  away  and  turn  our  attention  to  some  of  the  more 
pressing  problems  of  lunar  politics. 

WHO  KNOWS? 

Yet  before  doing  so  I  would  like  to  ask,  How  do  you  know 
that  our  mighty  Universe  is  not  a  similar  pebble  on  the  shore 
of  some  gigantic  world  ?  It  may  seem  to  some  that  absurdity 
could  go  no  further  than  this  suggestion.  Yet  it  is  a  surmise 
logically  based  on  the  generally  accepted  infra-atomic  "  star- 
clusters"  of  our  foremost  scientists.  As  such  I  cheerfully 
submit  it  to  them  for  their  prayerful  consideration.  If  their 
corpuscular  theory  of  atoms  is  true  —  and  I  should  be  the  last 
to  deny  it  —  then  this  celestial  application  of  it  may  be  true 
also.  Or  it  may  not.  ^  Quien  sale  ? 


THE   GREEK   ALPHABET  301 


APPENDIX  B 

THE   GREEK  ALPHABET 

THE  study  of  the  Star  Charts  requires  a  knowledge  of 
the  Greek   alphabet.     It  is  therefore   printed   here  for 
reference,  with  the  name  and  sound  of  each  letter. 

GREEK 
A  a 

B  j3 

r          y 

A  d 

E  s  or  e 

Z  f 

H  r) 

e  #or  6 

1  i 

K  K 

A  X 

M  11 

N  v 

*  £ 
O  o 

n         TT 
p          P 

2  a-  or  $• 
T  T 

Y  v 

*  0 
X 


NAME 

ENGLISH 

Alpha 
Beta 

a 
b 

Gamma 
Delta 

g 
d 

Epsilon 
Zeta 

u 
z 

Eta 

e 

Theta 

th 

Iota 

i 

Kappa 
Lambda 

k 
1 

Mu 

m 

Nu 

n 

Xi 

X 

Omicron 

6 

Pi 
Rho 

P 
r 

Sigma 
Tau 

s 
t 

Upsilon 
Phi 
Chi 

u 

ph 

ch 

Psi 

Omega 

ps 
o 

302     HOW  TO   KNOW  THE   STARRY   HEAVENS 

APPENDIX  C 

THE   LUNAR  CRATERS 
EXPLANATION  OF  THE  FIRST  FOUR  TABLES 

THE  number  before  the  name  of  a  crater  indicates  its  place  on 
the  charts. 

The  craters  which  have  no  number  before  them  are  lettered  on 
the  charts. 

Small  odd  numbers  (1  to  81)  indicate  craters  on  the  north  half  of 
the  western  chart. 

Large  odd  numbers  (83  to  169)  indicate  craters  on  the  south  half 
of  the  'western  chart. 

Small  even  numbers  (2  to  68)  indicate  craters  on  the  north  half  of 
the  eastern  chart. 

Large  even  numbers  (70  to  176)  indicate  craters  on  the  south  half 
of  the  eastern  chart. 

The  names  in  italics  indicate  craters  near  the  centre  of  the  lunar 
disc. 

The  names  in  roman  type  indicate  craters  near  the  circumference 
of  the  lunar  disc. 

Small  numbers  in  parentheses,  ( ),  denote  the  diameter  of  the 
crater  in  miles. 

Large  numbers  in  brackets,  [  ],  denote  the  extreme  height  of  the 
rampart  (in  feet)  above  the  floor  of  the  crater. 

TABLE  I 
CRATERS  ON  THE  NORTHWEST  QUARTER  OF  THE  MOON 

[See  Chart  F] 


1  Barrow  (40) 

3  Strabo 

5  Aristoteles  (60),  [11,000] 

7  Endymion  (80) 


9  Atlas  (55),  [11,000] 

11  Hercules  (46),  [11,000] 

13  Berg 

15  Eudoxus  (40) 


THE   LUNAR   CRATERS 


303 


17  Messala 

19  Cassini  (36),  [4,000] 

21  Gauss  (110) 

23  Berzelius 

25  Thesetetus 

27  Gemiims 

29  Aristillus  (34),  [11,000] 
Posidonius  (60),  [6,000] 

31  Autolycus  (23),  [4,000] 

33  Linne 

35  Burckhardt 

37  Tralles 

39  Cleomenes  (80),  [10,000] 

41  Roemer 

43  Macrobius  (40),  [13,000] 

45  Bessel 

47  Maraldi 

49  Sulpicius  Gallus 

51  Peirce 


53   Vitruvius 

Proclus 
55  Alhazen 
57  Dawes 

Menelaus  (20) 
59   Hansen 
61   Picard 

Plinius  (30),  [6,000] 

Manilius  (25) 
63   Condorcet 
65  Julius  Ccesar 
67   Firmicus 
69   Taruntius 
71    Silberschlag 
73   Hyginus 
75   Ariadceus 
77   Agrippa  (30) 
79    Triesnecker  (20) 
81    Godin  (22) 


TABLE   II 
CRATERS  ON  THE  SOUTHWEST  QUARTER  OF  THE  MOON 


[See  Chart  F] 


83  Webb 

85  Messier 

87  Delambre 

89  Hipparchus  (100) 

91  Hypatia 

93  Langrenus  (90),  [10,000] 

95  Torricelli 

97  Taylor 

99  Isadorus 

101  Guttemberg  (45) 

103  Goclenius  (28) 

Theophilus  (64),  [18,000] 

105  Cyrillus  (60) 

107  Albategnius  (65),  [15,000] 

109  Abulfeda 

111  Vendelinus 

113  Catharina  (65) 

115  Almamon 


117  Sacrobosco 

119  Santbeck 

121  Fracastorius 

123  Apianus 

125  Humboldt  [16,000] 

Petavius  (78) 

127  Pontanus 

129  Werner  (45) 

131  Aliacensis  [16,000] 

133  Piccolomini  [15,000] 

135  Zagut 

137  Reichenbach 

139  Walter  (100) 

141  Lindenau 

143  Gemma 

145  Frisius 

147  Neander 

149  Rabbi  Levi 


304     HOW  TO   KNOW  THE   STARRY   HEAVENS 


151  Stiborius 

153  Furnerius 

155  Rheita 

157  Maurolycus  (150),  [14,000] 

159  Stofler  (110) 


161  Fabricius 

163  Clairaut 

165  Pitiscus 

167  Bacon 

169  Curtius 


TABLE   III 
CRATERS  ON  THE  NORTHEAST  QUARTER  OF  THE  MOON 


[See  Chart  E] 


2  Philolaus 

4  Pythagoras 

6  Fontanelle 

8  Condamine 

Plato  (60),  [7,000] 

10  Harpalus 

12  Bianchini 

14  Sharp 

16  Leverrier 

18  Helicon 

20  Mairan 

22  Herschel(17) 

24  Gruithuisen 

26  Lichtenberg 

Archimedes  (52),  [4,000] 

28  Delisle 

30  Beer 

32  Timocharis  (23),  [7,000] 

34  Diophantus 

36  Lambert  (17) 


38   Euler  (19) 

Aristarchus  (28) 
40   Herodotus 
42   Pytheas 
44  Tobias  Mayer 
46   Eratosthenes  (37),  [16,000] 
48  Bessarion 
50  Marius 
52   Stadius 

Copernicus  (56),  [12,000] 
54   Galileo 

Kepler  (22) 
56   Reiner 
58   Schroeter 
60   Reinhold 
62  Kunowsky 
64  Encke 
66    Hevel 
68   Gambert 


TABLE  IV 
CRATERS  ON  THE  SOUTHEAST  QUARTER  OF  THE  MOON 


[See  Chart  E] 


70  Landsberg 
72  Mosting 
74   Lalande 


76 

78 
80 


Fra  Mauro 

Parry 

Bonpland 


_<    •       'Saiy   o/  Ctt*»£apk<e    'Pit A 
Rai™*  * 


CHART  E.  —  EASTERN  HALF  OF  MOON 


C=>c-_    C^>   <-> 
g?   -J  » 

^tA     or  i"       ^^%^°, 

«touV«iiL^K_\^^ 


•a«  ^.-V;P!S-.-  .«^i- 

^e-<ri 

D>      P        P        * 
r>  ••      '/     g         "? 

r»o        p^ 

<=4         W-*    ,O  ^ 

".    -J13    ''     n*«,     L     *  V1 


CHART  F.  —  WESTERN  HALF  OK  MOON 


THE   LUNAR   CRATERS 


305 


82  Flamsteed 

84  Damoiseau 
Grimaldi  (150) 

86  Euclides 

Ptolemy  (115) 

88  Guerike 

90  Letronne 

92  Hansteen 
Alphons  (83) 

94  A  Ipetragius 

96  Lassell 

98  Billy 

Arzachel  (65) 

100  Gassendi  (54) 

102  Fontana 

104  Thebit  (30) 

106  Birt 

108  Nicollet 

110  Bullialdus  (38), 

112  Agath  archides 

114  Mersenius  (40) 

116  Eichstadt 

118  Purbach  (60) 

120  Regiomontanus 

122  Pilatus 

124  Mercator 

126  Vitello  (24) 

128  Vieta 


[8,000] 


130  Hell 

132  Cichus 

134  Lexell 

136  Ball 

138  Capuanus 

140  Lagrange 

142  Lacroix 

144  Piazzi 

146  Bouvard 

148  Heinsius 

150  Saussure 

Tycho  (54),  [17,000] 

152  Hainzel 

Schickard  (134),  [9,000] 

154  Wilhelml. 

156  Inghirami 

158  Maginus  (100) 

Wargentin  (53) 

160  Longimontanus 

162  Schiller 

Clavius  (140),  [17,000] 

164  Scheiner 

166  Bettinus 

168  Bailly 

170  Kircher 

172  Moretus 

174  Casatus 

176  Newton  [24,000] 


TABLE  V 

ALPHABETICAL  LIST  OF  CRATERS 


Abulfeda,  109 
Agatharchides,  112 
Agrippa,  77 
Albategnius,  107 
Alhazen,  55 
Aliacensis,  131 
Almamon,  115 
Alpetragius,  94 
Alphons 
Apianus,  123 
Archimedes 
Ariadceus,  75 


Aristarchus 
Aristillus,  29 
Aristoteles,  5 
Arzachel 
Atlas,  9 
Autolycus,  31 

Bacon,  167 
Bailly,  168 
Ball,  136 
Barrow,  1 
Beer,  30 

20 


Berzelius,  23 
Bessarion,  48 
Bessel,  45 
Bettinus,  166 
Bianchini,  12 
Billy,  98 
Birt,  106 
Bonpland,  80 
Bouvard,  146 
Bullialdus,  110 
Burckhardt,  35 
Burg,  13 


306     HOW   TO   KNOW   THE   STARRY   HEAVENS 


Capuanus,  138 

Hainzel,  152 

Messala,  17 

Casatus,  174 

Hansen,  59 

Messier,  85 

Cassini,  19 

Hansteen,  92 

Moretus,  172 

Catharina,  113 

Harpalus,  10 

Hosting,  72 

Cichus,  132 

Heinsius,  148 

Clairaut,  163 

Helicon,  18 

Neander,  147 

Clavius 

Hell,  130 

Newton,  176 

Cleomenes,  39 

Hercules,  11 

Nicollet,  108 

Condamini,  8 

Herodotus,  40 

Condorcet,  63 

Herschel,  22 

Parry,  78 

Copernicus 

Hevel,  66 

Peirce,  51 

Cur-tins,  169 

Hipparchus,  89 

Petavius 

Cyrillus,  105 

Humboldt,  125 

Philolaus,  2 

Hyginus,  73 

Piazzi,  144 

Damoiseau,  84 

Hypatia,  91 

Picard,  61 

Dawes,  57 

Piccolomini,  133 

Delambre,  87 

Inghirami,  156 

Pilatus,  122 

Delisle,  28 

Isadorus,  99 

Pitiscus,  165 

Diophantus,  34 

Plato 

Julius  Ccesar,  65 

Plinius 

Eichstadt,  116 

Pontanus,  127 

Encke,  64 

Kepler 

Posidonius 

Endymion,  7 

Kircher,  170 

Proclus 

Eratosthenes,  46 

Kunowsky,  62 

Ptolemy 

Euclides,  86 

Purbach,  118 

Eudoxus,  15 

Lacroix,  142 

Pythagoras,  4 

Euler,  38 

Lagrange,  140 

Pytheas,  42 

Lalande,  74 

Fabricius,  161 

Lambert,  36 

Rabbi  Levi,  149 

Firmicus,  67 

Landsberg,  70 

Regiomontanus,  120 

Flamsteed,  82 

Langrenus,  93 

Reichenbach,  137 

Fontana,  102 

Lassell,  96 

Reiner,  56 

Fontanelle,  6 

Letronne,  90 

Reinhold,  60 

Fracastorius,  121 

Leverrier,  16 

Rheita,  155 

Fra  Mauro,  76 

Lexell,  134 

Roemer,  41. 

Frisius,  145 

Lichtenberg,  26 

Furnerius,  153 

Lindenau,  141 

Sacrobosco,  117 

Linne,  33 

Santbeck,  119 

Galileo,  54 

Longimontanus,  160 

Saussure,  150 

Gambert,  68 

Scheiner,  164 

Gassendi,  100 

Macrobius,  43 

Schickard 

Gauss,  21 

Maginus,  158 

Schiller,  162 

Geminus,  27 

Mairan,  20 

Schroeter,  58 

Gemma,  143 

Manilius 

Sharp,  14 

Goclenius,  103 

Maraldi,  47 

Silberschlag,  71 

Godin,  81 

Marius,  50 

Stadius,  52 

Grimaldi 

Maurolycus,  157 

Stiborius,  151 

Gruithuisen,  24 

Menelaus 

Stofler,  159 

Guerike,  88 

Mercator,  124 

Strabo,  3 

Guttemberg,  101 

Mersenius,  114 

Sulpicius  Gallus,  49 

THE   LUNAR   CRATERS 


307 


Taruntius,  69 
Taylor,  97 
Thesetetus,  25 
Thebit,  104 
Theophilus 
Timocharis,  32 
Tobias  Mayer,  44 
Torricelli,  95 

Tralles,  37 
Triesnecker,  79 
Tycho 

Vendelinus,  111 
Vieta,  128 
Vitello,  126 
Vitruvius,  53 

Walter,  139 
Wargentiu 
Webb,  83 
Werner,  129 
Wilhelm  I,  154 

Zagut,  135 


CHART  G.  — THE  C 


Astrological  He 
to  explain  t 

PERSIAN,  JEW 

RE 


P&le  Nord       -       - 

North  Pole. 

Voye  Lact£e 

Milky  Way. 

Hydre      -       - 

Hydra. 

Taureau 

Taurus;  Bull. 

Orion  ;  Nemrod      - 

Orion  ;  Nimrod. 

Gemeaus 

Gemini. 

Crab  ou  Cancer 

Crab  or  Cancer 

Bellier;  Agneau  de 

Ram  ;  Lamb  of  God  ; 

Dieu 

Aries. 

Pers^e;  Cherubin  - 

Perseus  ;  Cherubim. 

Etable  d'louseph 

Joseph's  Stable  ;  Auriga. 

Lion         - 

Lion;  Leo. 

Nortnern  Hemisphere. 

Translation. 


Ours ;  Sanglier ;  Ane  ; 

Typhon    - 
Poissons  - 
Andromede 
Dragon  des  Hesperides 
Vierge;  Eve;  Sybille; 

Isis,  &c. 

Bootes;  Adam;  Osiris; 
Couronne 
Hercule 
Serpent  d'Eve ;  Ahri- 

man ;  Satan 


Bear ;  Boar ;  Ass ; 

Typhon. 

Pisces  —  the  Fishes. 
Andromeda. 

Dragon  of  the  Hesperides 
Virgin  ;  Eve  ;  Sybil ; 

Isis,  &c. 

Bootes ;  Adam  ;  Osiris. 
Corona  Borealis. 
Hercules. 
Eve's  Serpent ;  Ahri- 

manes;  Serpentinus. 


STELLATION    FIGURES. 


of  the 
of  the  Ancients 

ryslertes  of  the 

AND  CHRISTIAN 
ONS. 


Southern  Hemisphere. 

Translation, 


hien  ;  Sirius 

Dog;  Sirius. 

Corbeau  de  No£ 

Noah's  Raven  ;  Corvus 

ridan       ... 

Eridanus. 

Verseau    ... 

Aquarius,  the  Water- 

aleine         ... 

Whale  ;  Cetus. 

bearer. 

il     -        -        - 

Nile. 

Capricorne 

Capricornus. 

oupe           ... 

Crab;  Cup. 

Sagittaire 

Sagittarius,  the  Archer 

aisseau  ;  Argo  ;  Arche 

Vessel  ;  Argo  ;  Navis  ; 

Voye  Lact6e 

Milky  Way. 

Ark. 

Scorpion  - 

Scorpio. 

uiopus       - 

Canopus. 

Balance 

Scales;  Libra. 

.le  Sud           -        - 

South  Pole. 

INDEX 


ABSORPTION  or  STARLIGHT,  84 

Absorption  spectra,  92 

Airy,  Sir  George,  26,  39 

Aldebaran,  103 

Algol,  108 

Allen,  Dr.  F.  J.,  273 

Alpha  Centauri,  53,  78 

Alpha  Draconis,  147 

Alphonso,  King,  20 

Alt-azimuth  telescope,  153 

Andromeda,  nebula  in,  122 

Angles,  measurement  of,  31,  39 

Angular  diameter,  41 

Aphelion,  160 

Apocalypse,  17,  26,  239 

Apparent  motions,  4 

Appearances,  superficial,  1 

Aratus,  118 

Arc,  chord  of,  41 ;  of  circle,  40 

Arc  problems,  47 

Aries,  First  Point  of,  10,  148 

Aristarchus,  lunar  crater,  264 

Aristotle,  163 

Ascension,  right,  149 

Asteroids,  29,  61,  64 

Astronomy,  Egyptian,  16;   Greek,  16, 

42 ;  Hindu,  23,  43 
Atomic  theory,  192 
Autumnal  equinox,  7,  139 

BALANCE,  TORSION,  38 
Base  line,  31,  32,  34 
Becquerel  rays,  197 
Bede,  Venerable,  17 
Beta  Lyrae,  108 
Bifrost,  Bridge  of,  87 
Binaries,  108,  110 
Blood  discs,  stack  of,  73 
Brahe,  Tycho,  19 


Bridge  of  Bifrost,  87 
Byron,  Lord,  188 

CALCIUM  FLOCCOLI,  130 

"  Canals  "  of  Mars,  226 

Cannon-ball,  speed  of,  71 

Canopus,  104 

Canopy  theory,  16 

Celestial    distances,    measurement    of, 

32 

Celestial  equator,  7,  119 
Celestial  latitude  and  longitude,  150 
Chariot  of  imagination,  53,  136,  220 
Chord  of  arc,  41 

Chromosphere,  or  sierra,  60,  129 
Circle,  arc  of,  40;  meridian,  152 
Clavius,  lunar  crater,  253 
Clouds,  Magellanic,  122 
Clusters  of  stars,  1 1 1 
Colliding  stars,  110,  206,  237 
Colours  of  stars,  98,111 
Comets,  221 

Comparisons,  planetary,  69,  77 
Constellations,  115,  137 
Copernican  system,  24  . 

Copernicus,  lunar  crater,  263 
Corona,  60,  131,  176 
Corpuscles,  or  negative  particles,  89, 199 
Craters,  lunar,  245,  253,  254,  264 
Creation,  18 
Creationism,  180 
Crookes,  Sir  William,  179,  196 
Crystal  spheres,  18 

DARK  STARS,  107,  135 
Darwin,  Charles,  201 
Darwin,  George,  204,  213 
Declination,  north  and  south,  149 
DeQuincey,  Thomas,  77 


310 


INDEX 


Dipper,  the,  14 

Distances,  estimating,  30  ;  measurement 
of  celestial,  32 ;  measurement  of  in- 
accessible, 31,  43  ;  of  stars,  123 

Distribution  of  stars  and  nebulae,  114, 
121 

Diurnal  motion,  4 

Double  stars,  108,  110;  spectroscopic, 
108 

Draco,  117 

Draper,  Dr.  John  W.,  22,  156,  178,  265, 
283 

Drift,  solar,  111 ;  stellar,  113,  147 

Duffield,  Prof.,  174 

EARTH,  62,  69,  139  ;  centre  of  system, 
15  ;  measuring  the,  31 ;  revolution  of, 
23,  138;  rotation  of,  23,  138;  shape 
of,  3 ;  weight  of,  38 

"  Ecclesiastes,"  286 

Eclipses,  11,  146 

Ecliptic,  8,  9,  119,  138 

Egyptian  astronomy,  16 

Electro-magnetic  theory,  Maxwell's, 
195 

Electronic  theory,  200 

Elements,  in  sun,  1 29  ;  periodic  system 
of,  192 

Ellipse,  158 

Equator,  celestial,  7,  119 

Equatorial  telescope,  12,  150 

Equinoxes,  7,  139;  precession  of,  141, 
174 

Eros,  33 

Estimating  distances,  30 

Ether,  89,  171,  191,  195 

Evolution  of  organic  life,  201,  274 

Evolution  of  solar  system,  208 

Evolution,  tidal,  202,  213 

FACUL,E  AND  FLOCCULI,  127,  130 
"  Faust,"  Goethe's,  63 
"Faustus,"  Marlowe's,  21,  136 
Fibre,  quartz,  75 
Firmament,  massive,  15,  16 
Flames,  solar,  59,  130,  176 
Flat-world  ideas,  15 
Fleming,  Prof.  J.  A.,  190 


Flocculi  and  facute,  127,  130 
Fraunhofer,  Joseph  von,  194 

GALAXY,  the,  56,  120 

Galileo,  169, 258;  his  laws  of  motion,  164 

"  Genesis,"  15,  82,  87 

George,  Henry,  155,  163 

Goethe's  "  Faust,"  63 

Gravitation,  law  of,  167 

Great  Bear,  14,  120 

Great  Pyramid,  147 

Greek  astronomy,  16,  42 

HAECKEL,  ERNST,  174,  207,  217 
Helium,  197 

Hercules,  cluster  in,  111 
Herschel,  Sir  William,  185 
Hindu  astronomy,  23,  43 
Holden,  Dr.  Edward  S.,  22 
Human  laws,  156 

INERTIA,  164 

"Jos,"  17 

Jupiter,  27,  64,  69,  233 

KANT,  IMMANUEL,  184 

Kathode  rays,  196 
Keeler,  Prof.  James  E.,  122 
Kepler,  Johann,  258;  Laws  of,  157 
Kinetic  theory  of  substance,  194 
Kirchhoff,  Gustav  R.,  194 
Koran,  the,  17 

LAND-SURVEYING,  31 

Laplace,  Pierre,  184 

Latitude,  celestial,  150 

Lavoisier,  Antoine  L.,  193 

Law  of  gravitation,  167 

Law  of  substance,  192 

Laws,  human,  156;  of  motion,  164;  of 

nature,  155 

Life,  evolution  of  organic,  201,  274 
Light,  refraction  of,  89 ;  speed  of,  54, 

72 
Light  waves,  lengthening  of,  84 ;  violet, 

73,  91 
Light  years,  68 


INDEX 


311 


Line  of  sight,  motion  in,  100 

Locomotive,  speed  of,  84 

Lodge,  Sir  Oliver,  200 

Longitude,  celestial,  150 

Lowe,  Geo.  N.,  85,  156,  286,  293 

Lunar  craters,  245,  253,  263,  264 

Lunar  mountains,  260 

Lunar  plains,  245 

Luther,  Martin,  23 

Lyre,  constellation  of,  112 

MAGELLANIC  CLOUDS,  122 

Magnitude,  stellar,  102,  103 

Marlowe's  "Faustus,"  21,  136 

Mars,  27,  63,  69  ;  "  canals  "  of,  226 

Mass  and  weight,  36 

Massive  firmament,  15,  16 

Matter,  89,  191,  295;  pyknotic  theory 
of,  195 

Maunder,  E.  W.,  118,  120 

Maxwell's  electro-magnetic  theory, 
195 

Mayer,  Robert,  193 

Measurement,  of  angles,  31,39  ;  of  celes- 
tial distances,  32 ;  of  inaccessible  dis- 
tances, 31,  43  ;  of  the  earth,  31 

Mercury,  27,  69 

Meridian  circle,  152 

Meteorites,  220,  230 

Meteors,  230 

Midsummer  solstice,  8 

Milky  Way,  56,  120 

Millikan,  R.  A.,  198 

Milton,  John,  53,  120,  223,  224,  243, 
271 

Mira  Ceti,  108 

Moon,  the,  244;  birth,  257;  distance, 
45;  motions,  11,  23;  size  and  mass, 
35 

Morris,  George,  280 

Motion,  diurnal,  4 ;  in  line  of  sight, 
100;  laws  of,  164;  of  sun,  6,  7 

Motions,  apparent,  4 

Mural  circle,  1 53 

NADIR,  5 

Nasmyth,  James,  257 

Nature,  laws  of,  155 


Nebulae,  12  J -123;  classes  of,  97;  dis- 
tribution of  stars  and,  114,  121; 
planetary,  97 

Nebular  hypothesis,  184,  207 

Negative  particles,  89,  199 

Neptune,  66,  69 

New  stars,  109 

Newcomb,  Prof.  Simon,  68,  77,  84,  114 

Newton,  Sir  Isaac,  124,  162;  his  law  of 
gravitation,  167 

North  and  south  declination,  149 

Nutation,  175 

OBLIQUITY  OF -ECLIPTIC,  8,  138 
Organic  life,  evolution  of,  201,  274 
Orientation  of  pyramid,  147 
Orion  nebula,  123 

PAPER,  pile  of,  72 ;  roll  of,  77 

Parallax,  44 

Particles,  negative,  89,  199 

Pascal,  Blaise,  84 

Passover,  spring,  7,  139 

Perihelion,  140,  160 

Periodic  system  of  elements,  192 

Perturbations  of  planets,  171 

Photographing  stars,  83 

Photosphere  of  sun,  59,  126 

Planetary  comparisons,  69,  77 

Planetary  nebulae,  97 

Planets,  3,  61,  211;  comparative  dis- 
tances, 27,  33,  77  ;  density,  mass,  and 
size,  35,  36 ;  inferior  and  superior, 
27,  28,  61 ;  motions  of,  11,  138;  move 
in  ellipses,  29,  157 ;  order  of,  26 ; 
perturbations  of,  171;  real  distances 
of,  29,  33,  68 

Plato,  lunar  crater,  263 

Pleiades,  the,  103,  120 

Pointers,  the,  14 

Polaris,  14,  103,  111 

Pole-stars,  different,  141,  147 

Poles  of  rotation,  140 

Precession  of  equinoxes,  141,  174 

Prisms,  87 

Proctor,  Richard  A.,  85,  118,  124,  125, 
288 

Prominences,  solar,  59,  130,  176 


312 


INDEX 


"  Psalms,"  the,  57 
Ptolemaic  system,  1 8 
Ptolemy,  42,  118 
Pyknotic  theory  of  matter,  195 
Pyramid,  the  Great,  147 
Pythagoras,  23 

QUARTZ  FIBRE,  75 

RADIAL  MOTIONS,  100 

Radian,  40 

Radiant  energy  of  sun,  133 

Radiation,  spectrum,  92 

Radium,  198 

Radius  vector,  160 

Railroad  to   Neptune,   71 ;    to  nearest 

star,  75 
Rainbow,  87 

Rays,  Becquerel,  197 ;  kathode,  196 
Red  light-waves,  91 
Refraction  of  light,  89 
Repulsion,  176 
Reversing  layer  of  sun,  129 
Revolution  of  the  earth,  23,  138 
Revolving  wheel,  75 
Right  Ascension,  149 
Ring  nebula  in  Lyra,  122 
Rontgen  rays,  196 
Rotation,   of    the  earth,    23,    138 ;   of 

planets,  138,  209;  of  sun,  129;  poles 

of,  140 

SATELLITES,  or  Moons,  61,  62,  65 

Saturn,  27,  64,  69 

Schaeberli,  Prof.,  122 

Schiaparelli,  266 

"  Seasons,  The,"  229 

Seasons  on  the  earth,  139 

See,  Dr.  J.  J.,  84,  101 

Serviss,  Garrett  P.,  120 

Shakespeare,  140 

Shelley,  Percy  B.,  57,  205 

Shooting  stars,  230 

Sierra,  or  chromosphere,  60,  129 

Sight,  motion  in  line  of,  100 

Signs  of  zodiac,  10,  137 

Sine  of  angle,  43 

Sine  problems,  49 

Sirius,  68,  78,  103 


Solar  flames  or  prominences,  59,  130, 
176 

Solar  system,  drift  of,  111  ;  evolution 
of,  208 

Solstice,  midsummer,  8;  winter,  140 

Spectroscope,  88,  194 

Spectroscopic  binaries,  108 

Spectrum,  88 ;  continuous,  92 ;  of  nebu- 
lae, 96;  stellar,  95;  of  star-clusters, 
96 

Spectrum,  absorption,  92 

Spectrum  analysis,  93 

Spectrum,  emission,  or  radiation,  92 

Spectrum,  flash,  of  sun,  129 

Spencer,  Herbert,  201 

Spheres,  crystal,  18;  umbrella,  4 

Spider's  thread,  74 

Spiral  nebulas,  98,  122 

Spots  in  the  sun,  127 

Spring  passover,  7,  139 

Starlight,  absorption  of,  84 

Stars,  3  ;  beyond  planets,  26 ;  classes 
of,  98;  clusters  of,  111;  colliding, 
110,  206,  237;  colours  of,  98,  111; 
dark,  107,  135;  distances  of,  23;  dis- 
tribution of,  114,  121;  double,  108, 
110;  drift  of,  113,  147;  magnitudes, 
102,  103  ;  nearest,  34,  68 ;  new,  109 ; 
number  of,  82 ;  photographing,  83 ; 
telescopic,  83 ;  trails,  13;  variable, 
107 ;  visible,  55,  82 

Steele,  J.  D.,  167 

Stellar  drift,  113,  147 

Sterling,  George,  53,  84,  102,  109,  126, 
205,  281,  282 

Substance,  law  of,  192;  theories  of, 
194,  195 

Sun,  annual  motion,  7  ;  bulk  and  mass, 
37  ;  chromosphere  or  sierra,  60,  129; 
corona,  60,  131,  176  ;  daily  motion, 
6 ;  diameter,  35 ;  distance,  32 ;  ele- 
ments in,  129;  faculae  and  flocculi, 
127,  130;  flames,  59,  130,  176;  inte- 
rior, 125,  134  ;  photosphere  or  visible 
surface,  59,  126;  prominences  or 
solar  flames,  59,  130,  176;  radiant 
energy,  133 ;  -reversing  layer,  129; 
rotation,  129;  spots,  127 


INDEX 


313 


Suns,  colliding,  110,  206,  237 
Superficial  appearances,  1 

TELESCOPE  MOUNTINGS,  alt-azimuth, 
153;  equatorial,  12,  150;  meridian 
circle,  152  ;  mural  circle,  153 ;  transit 
instrument,  153 

Telescopic  stars,  83 

Tele-spectroscope,  95 

Tennyson,  Alfred,  110 

"  Testimony  of  the  Suns,"  53,  84,  102, 
109,  126,  205,  281,  282 

Theophilus,  lunar  crater,  253 

Thomson,  Prof.,  196 

Thomson,  James,  "  The  Seasons,"  229 

Thread,  quartz,  75 ;  spider's,  74 ;  a 
strand  of,  74 

Tidal  evolution,  202,  213 

Time,  difference  in,  5,  12 

Torsion  balance,  38 

Trails,  star,  13 

Transit  instrument,  153 

Triangulation,  31 

Trifid  Nebula,  123 

Trigonometry,  39 

Tycho  Brahe,  19 


UMBRELLA  SPHERES,  4 

Universes,  outside,  86 
Uranus,  66,  69 

VARIABLE  STARS,  107 

Vega,  112 

Venerable  Bede,  17 

Venus,  27,  69 

Vernal  equinox,  7,  139 

Via  Lactea,  120 

Vibratory  theory  of  substance,  195 

Violet  light- waves,  73,  91 

WEIGHT  AND  MASS,  36 
Wheel,  revolving,  75 
Winter  solstice,  140 
Wollaston,  194 

YOUNG,  PROF.  CHARLES  A.,  131 

Yule-tide  solstice,  8 

ZAZEL,  A.,  ix.  219,  233,  242,  290 

Zenith,  5 

Zodiac,  signs  of  the,  10,  137 


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